This question tests 2nd grade fluency in adding and subtracting within 20, meaning solving these facts quickly and accurately using mental strategies or automatic recall (CCSS 2.OA.B.2: Fluently add and subtract within 20 using mental strategies. By end of Grade 2, know from memory all sums of two one-digit numbers). Fluency means answering quickly and correctly, ideally within a few seconds. Strategies for fluency include: (1) Make ten—decompose to make 10 first (8+7 → 8+2+5 → 10+5=15); (2) Doubles—know doubles facts (7+7=14), use for near doubles (7+8: one more than 7+7, so 15); (3) Think addition for subtraction (15-8: think '8 plus what equals 15? 8+7=15, so answer is 7'); (4) Fact families—if know 8+7=15, then know 15-7=8 and 15-8=7; (5) Memorization—automatic recall of facts. By end of 2nd grade, students should know these facts from memory. In this problem, student must subtract 13-8. To solve fluently, use think addition for subtraction (13-8: think '8 plus what equals 13? 8+5=13, so answer is 5'), or recall from memory (automatic: 13-8=5). Choice A is correct because subtracting 13-8 correctly equals 5 (verified by thinking addition: 8+5=13, so 13-8=5). This demonstrates fluency—quick, accurate calculation within 20. Choice C represents a common error like off by one (said 8 instead of 5, perhaps confusing with the subtrahend). This error typically happens when students miscount or confuse subtraction direction. To help students achieve fluency: Practice facts daily—quick drills, flashcards, games, timed exercises (with emphasis on improvement, not pressure). Teach strategies explicitly: make ten (show with ten frames or fingers: 8 fingers up, need 2 more to fill 10, take 2 from 7 leaving 5, now 10+5=15), doubles (memorize 6+6, 7+7, 8+8, 9+9), near doubles (one more/less than double). Build fact families: if practice 8+5=13, also practice 5+8=13, 13-5=8, 13-8=5—reduces facts to learn. Use games: card games, online fact practice, partner quizzes. Connect strategies: 'You can solve 13-8 by thinking: what plus 8 equals 13? We know 8+5=13, so 13-8=5.' Spiral review: keep practicing previously learned facts while introducing new ones. Celebrate progress toward automatic recall: 'You answered in 2 seconds—that's fluency!' Identify which facts each student still needs to work on, target practice. Use manipulatives initially (counters, ten frames) but fade toward mental calculation. Watch for: slow counting strategies (need to build toward recall), basic fact errors (13-8≠8), confusion between addition and subtraction, not using efficient strategies, giving up without trying mental strategies.

This question tests 2nd grade fluency in adding and subtracting within 20, meaning solving these facts quickly and accurately using mental strategies or automatic recall (CCSS 2.OA.B.2: Fluently add and subtract within 20 using mental strategies. By end of Grade 2, know from memory all sums of two one-digit numbers). Fluency means answering quickly and correctly, ideally within a few seconds. Strategies for fluency include: (1) Make ten—decompose to make 10 first (8+7 → 8+2+5 → 10+5=15); (2) Doubles—know doubles facts (7+7=14), use for near doubles (7+8: one more than 7+7, so 15); (3) Think addition for subtraction (15-8: think '8 plus what equals 15? 8+7=15, so answer is 7'); (4) Fact families—if know 8+7=15, then know 15-7=8 and 15-8=7; (5) Memorization—automatic recall of facts. By end of 2nd grade, students should know these facts from memory. In this problem, the student must subtract 15-8. To solve fluently, use think addition for subtraction (15-8: think '8 plus what equals 15? 8+7=15, so answer is 7'), or recall from memory (automatic: 15-8=7). Choice B is correct because subtracting 15-8 correctly equals 7 (verified by thinking addition: 8+7=15, so 15-8=7). Choice A represents 8, a subtraction direction error (15-8: calculated 8-15 incorrectly or gave 8 instead of 7). This error typically happens when students confuse the order of subtraction or don't use efficient strategies. To help students achieve fluency: Practice facts daily—quick drills, flashcards, games, timed exercises (with emphasis on improvement, not pressure). Teach strategies explicitly: make ten (show with ten frames or fingers: 8 fingers up, need 2 more to fill 10, take 2 from 7 leaving 5, now 10+5=15), doubles (memorize 6+6, 7+7, 8+8, 9+9), near doubles (one more/less than double). Build fact families: if practice 8+7=15, also practice 7+8=15, 15-7=8, 15-8=7—reduces facts to learn. Use games: card games, online fact practice, partner quizzes. Connect strategies: 'You can solve 15-8 by thinking: what plus 8 equals 15? We know 8+7=15, so 15-8=7.' Spiral review: keep practicing previously learned facts while introducing new ones. Celebrate progress toward automatic recall: 'You answered in 2 seconds—that's fluency!' Identify which facts each student still needs to work on, target practice. Use manipulatives initially (counters, ten frames) but fade toward mental calculation. Watch for: slow counting strategies (need to build toward recall), basic fact errors (8+7≠16), confusion between addition and subtraction, not using efficient strategies, giving up without trying mental strategies.

This question tests 2nd grade fluency in adding and subtracting within 20, meaning solving these facts quickly and accurately using mental strategies or automatic recall (CCSS 2.OA.B.2: Fluently add and subtract within 20 using mental strategies. By end of Grade 2, know from memory all sums of two one-digit numbers). Fluency means answering quickly and correctly, ideally within a few seconds. Strategies for fluency include: (1) Make ten—decompose to make 10 first (8+7 → 8+2+5 → 10+5=15); (2) Doubles—know doubles facts (7+7=14), use for near doubles (7+8: one more than 7+7, so 15); (3) Think addition for subtraction (15-8: think '8 plus what equals 15? 8+7=15, so answer is 7'); (4) Fact families—if know 8+7=15, then know 15-7=8 and 15-8=7; (5) Memorization—automatic recall of facts. By end of 2nd grade, students should know these facts from memory. In this problem, find the missing number in 16-=8. To solve fluently, use think addition (what plus 8 equals 16? 8+8=16, so 16-8=8), or recall from memory. Choice C is correct because for 16-=8, the answer is 8 since 16-8=8. Choice D represents 9, an off-by-one error (said 9 instead of 8—perhaps confused with 16-7=9). This error typically happens when students make strategy mistakes or lack automatic recall. To help students achieve fluency: Practice facts daily—quick drills, flashcards, games, timed exercises (with emphasis on improvement, not pressure). Teach strategies explicitly: make ten (show with ten frames or fingers: 8 fingers up, need 2 more to fill 10, take 2 from 7 leaving 5, now 10+5=15), doubles (memorize 6+6, 7+7, 8+8, 9+9), near doubles (one more/less than double). Build fact families: if practice 8+7=15, also practice 7+8=15, 15-7=8, 15-8=7—reduces facts to learn. Use games: card games, online fact practice, partner quizzes. Connect strategies: 'You can solve 15-8 by thinking: what plus 8 equals 15? We know 8+7=15, so 15-8=7.' Spiral review: keep practicing previously learned facts while introducing new ones. Celebrate progress toward automatic recall: 'You answered in 2 seconds—that's fluency!' Identify which facts each student still needs to work on, target practice. Use manipulatives initially (counters, ten frames) but fade toward mental calculation. Watch for: slow counting strategies (need to build toward recall), basic fact errors (8+7≠16), confusion between addition and subtraction, not using efficient strategies, giving up without trying mental strategies.

This question tests 2nd grade fluency in adding and subtracting within 20, meaning solving these facts quickly and accurately using mental strategies or automatic recall (CCSS 2.OA.B.2: Fluently add and subtract within 20 using mental strategies. By end of Grade 2, know from memory all sums of two one-digit numbers). Fluency means answering quickly and correctly, ideally within a few seconds. Strategies for fluency include: (1) Make ten—decompose to make 10 first (8+7 → 8+2+5 → 10+5=15); (2) Doubles—know doubles facts (7+7=14), use for near doubles (7+8: one more than 7+7, so 15); (3) Think addition for subtraction (15-8: think '8 plus what equals 15? 8+7=15, so answer is 7'); (4) Fact families—if know 8+7=15, then know 15-7=8 and 15-8=7; (5) Memorization—automatic recall of facts. By end of 2nd grade, students should know these facts from memory. In this problem, the student must add 9+6. To solve fluently, use make ten strategy (9+6: need 1 more to make 10, so break 6 into 1+5, then 9+1=10, 10+5=15), or use doubles (know 6+6=12, but for 9+6 think of it as near ten: 10+5=15 but adjust since 9 is one less, actually 15-1=14 wait, better: automatic recall 9+6=15). Choice C is correct because adding 9+6 correctly equals 15 (can verify with make ten: 9+1=10, 10+5=15, or count on: 9+6 is 10,11,12,13,14,15). Choice A represents 14, an off-by-one error (said 14 instead of 15—counting error or basic fact mistake). This error typically happens when students miscount while adding on or confuse similar facts like 9+5=14. To help students achieve fluency: Practice facts daily—quick drills, flashcards, games, timed exercises (with emphasis on improvement, not pressure). Teach strategies explicitly: make ten (show with ten frames or fingers: 9 fingers up, need 1 more to fill 10, take 1 from 6 leaving 5, now 10+5=15), doubles (memorize 6+6, 7+7, 8+8, 9+9), near doubles (one more/less than double). Build fact families: if practice 9+6=15, also practice 6+9=15, 15-6=9, 15-9=6—reduces facts to learn. Use games: card games, online fact practice, partner quizzes. Connect strategies: 'You can solve 15-9 by thinking: what plus 9 equals 15? We know 9+6=15, so 15-9=6.' Spiral review: keep practicing previously learned facts while introducing new ones. Celebrate progress toward automatic recall: 'You answered in 2 seconds—that's fluency!' Identify which facts each student still needs to work on, target practice. Use manipulatives initially (counters, ten frames) but fade toward mental calculation. Watch for: slow counting strategies (need to build toward recall), basic fact errors (9+6≠14), confusion between addition and subtraction, not using efficient strategies, giving up without trying mental strategies.

This question tests 2nd grade fluency in adding and subtracting within 20, meaning solving these facts quickly and accurately using mental strategies or automatic recall (CCSS 2.OA.B.2: Fluently add and subtract within 20 using mental strategies. By end of Grade 2, know from memory all sums of two one-digit numbers). Fluency means answering quickly and correctly, ideally within a few seconds. Strategies for fluency include: (1) Make ten—decompose to make 10 first ($8+7 \rightarrow 8+2+5 \rightarrow 10+5=15$); (2) Doubles—know doubles facts ($7+7=14$), use for near doubles ($7+8$: one more than $7+7$, so $15$); (3) Think addition for subtraction ($15-8$: think '8 plus what equals 15? $8+7=15$, so answer is 7'); (4) Fact families—if know $8+7=15$, then know $15-7=8$ and $15-8=7$; (5) Memorization—automatic recall of facts. By end of 2nd grade, students should know these facts from memory. In this problem, solve word problem with 9 stickers +7 more. To solve fluently, use make ten strategy ($9+7$: need 1 more to make 10, so break 7 into 1+6, then $9+1=10$, $10+6=16$), or near doubles ($8+8=16$, adjust for $9+7=16$). Choice B is correct because 9 stickers +7 more =16 total (can verify with make ten: $9+1=10$, $10+6=16$, or automatic recall: $9+7=16$). This demonstrates fluency—quick, accurate calculation within 20. Choice A represents a common error like wrong operation (word problem said 'got more' but subtracted, getting 15 or other). This error typically happens when students use wrong operation or make counting errors. To help students achieve fluency: Practice facts daily—quick drills, flashcards, games, timed exercises (with emphasis on improvement, not pressure). Teach strategies explicitly: make ten (show with ten frames or fingers: 9 fingers up, need 1 more to fill 10, take 1 from 7 leaving 6, now $10+6=16$), doubles (memorize $6+6$, $7+7$, $8+8$, $9+9$), near doubles (one more/less than double). Build fact families: if practice $9+7=16$, also practice $7+9=16$, $16-7=9$, $16-9=7$—reduces facts to learn. Use games: card games, online fact practice, partner quizzes. Connect strategies: 'You can solve $16-9$ by thinking: what plus 9 equals 16? We know $9+7=16$, so $16-9=7$.' Spiral review: keep practicing previously learned facts while introducing new ones. Celebrate progress toward automatic recall: 'You answered in 2 seconds—that's fluency!' Identify which facts each student still needs to work on, target practice. Use manipulatives initially (counters, ten frames) but fade toward mental calculation. Watch for: slow counting strategies (need to build toward recall), basic fact errors ($9+7\neq15$), confusion between addition and subtraction, not using efficient strategies, giving up without trying mental strategies.

This question tests 2nd grade fluency in adding and subtracting within 20, meaning solving these facts quickly and accurately using mental strategies or automatic recall (CCSS 2.OA.B.2: Fluently add and subtract within 20 using mental strategies. By end of Grade 2, know from memory all sums of two one-digit numbers). Fluency means answering quickly and correctly, ideally within a few seconds. Strategies for fluency include: (1) Make ten—decompose to make 10 first ($8+7 \rightarrow 8+2+5 \rightarrow 10+5=15$); (2) Doubles—know doubles facts ($7+7=14$), use for near doubles ($7+8$: one more than $7+7$, so 15); (3) Think addition for subtraction ($15-8$: think '8 plus what equals 15? $8+7=15$, so answer is 7'); (4) Fact families—if know $8+7=15$, then know $15-7=8$ and $15-8=7$; (5) Memorization—automatic recall of facts. By end of 2nd grade, students should know these facts from memory. In this problem, student must subtract $16-9$. To solve fluently, use think addition for subtraction ($16-9$: think '9 plus what equals 16? $9+7=16$, so answer is 7'), OR use fact families (know $9+7=16$, so $16-9=7$), OR recall from memory (automatic: $16-9=7$). Choice B is correct because subtracting $16-9$ correctly equals 7 (verified by thinking addition: $9+7=16$, so $16-9=7$). This demonstrates fluency—quick, accurate calculation within 20. Choice A represents off by one (said 8 instead of 7—counting error or basic fact mistake). This error typically happens when students miscount or confuse with nearby facts like $16-8=8$. To help students achieve fluency: Practice facts daily—quick drills, flashcards, games, timed exercises (with emphasis on improvement, not pressure). Teach strategies explicitly: make ten (show with ten frames or fingers: 8 fingers up, need 2 more to fill 10, take 2 from 7 leaving 5, now $10+5=15$), doubles (memorize $6+6$, $7+7$, $8+8$, $9+9$), near doubles (one more/less than double). Build fact families: if practice $8+7=15$, also practice $7+8=15$, $15-7=8$, $15-8=7$—reduces facts to learn. Use games: card games, online fact practice, partner quizzes. Connect strategies: 'You can solve $15-8$ by thinking: what plus 8 equals 15? We know $8+7=15$, so $15-8=7$.' Spiral review: keep practicing previously learned facts while introducing new ones. Celebrate progress toward automatic recall: 'You answered in 2 seconds—that's fluency!' Identify which facts each student still needs to work on, target practice. Use manipulatives initially (counters, ten frames) but fade toward mental calculation. Watch for: slow counting strategies (need to build toward recall), basic fact errors ($8+7\neq16$), confusion between addition and subtraction, not using efficient strategies, giving up without trying mental strategies.

To find 18 - 6, we subtract: 18 - 6 = 12. Choice A (10) comes from miscounting steps in the sequence. Choice B (11) is an off-by-one error. Choice D (14) comes from adding instead of subtracting: 18 - 6 ≠ 6 + 8.

In the pattern, the first number decreases by 1 each time (15, 14, 13, 12) and the second number also decreases by 1 each time (8, 7, 6, 5), while the difference stays 7. So 12 - 5 = 7. Choice A has the wrong difference. Choice C skips 12. Choice D skips two numbers.

Carlos starts with 6 + 7 = 13 marbles total. When he trades 4 red marbles for 4 green marbles, he still has the same total number of marbles: 13. Choice A (9) incorrectly subtracts the traded marbles. Choice B (11) subtracts 2 from the original total. Choice D (17) incorrectly adds the 4 green marbles to the original total.

If two numbers sum to 16 and one is 9, then the other is 16 - 9 = 7. The larger number is 9 and the smaller is 7. So 9 - 7 = 2. Choice B (7) is the smaller number itself. Choice C (9) is the larger number itself. Choice D (16) is the sum of both numbers.