Fluently Add and Subtract Within 20 - 2nd Grade Math

1. What is $18 - 6$?
   Answer: $12$ Subtracting 6 from 18 gives 12, so C is correct. A is incorrect because it is too small to be the correct difference. B is incorrect because it is also too small, off by one from the correct answer. D is incorrect because it is too large to be the correct difference.
2. What is $9 + 8$?
   Answer: 17 Adding 9 and 8 gives 17. Choice B is 1 more than the correct sum. Choice C comes from doubles confusion, treating the problem like 8 plus 8. Choice D is 2 less than the correct sum.
3. If $7 + 8 = 15$, what is $15 - 7$?
   Answer: 8 Subtracting, $15 - 7 = 8$, so C is correct. Choice A repeats one of the numbers from the original fact instead of solving the new problem. Choice B repeats the other number being subtracted instead of the result. Choice D is two less than the correct difference, likely from a counting slip.
4. What is $7 + 8$?
   Answer: 15 7 plus 8 equals 15. 16 is 1 too high for the correct sum. 13 is 2 too low for the correct sum. 14 is 1 too low for the correct sum.
5. Solve $13 - 8$.
   Answer: 5 13 minus 8 equals 5, which is the correct difference. Choosing 6 or 4 reflects a small counting or subtraction error near the correct answer. Choosing 8 mistakenly repeats the subtrahend instead of solving for the difference.
6. What is $9 + 6$?
   Answer: 15 Adding 9 and 6 gives 15. Choice A, 13, undercounts by two. Choice C, 14, is off by one below the correct total. Choice D, 16, is off by one above the correct total.
7. Calculate $8 + 5$.
   Answer: 13 8 plus 5 equals 13, since counting on 5 from 8 gives 9, 10, 11, 12, 13. 15 comes from adding more than 5, such as mistaking the second number for 7. 12 comes from stopping one count too early. 14 comes from counting on but landing one count too high.
8. What is $15 - 8$?
   Answer: 7 The correct answer is 7, because $15 - 8 = 7$. Choice A (6) comes from subtracting one too many. Choice B (15) repeats the starting number instead of subtracting. Choice D (8) repeats the number being subtracted instead of finding the difference.
9. $16 - \underline{\hspace{2em}} = 8$. What number goes in the blank?
   Answer: 8 16 minus 8 equals 8, so the missing number is 8. 16 is the starting number, not the number being subtracted. 7 would make the equation equal 9, not 8. 6 would make the equation equal 10, not 8.
10. What number completes the equation $8 + n = 15$?
    Answer: 7 The missing number is 7, because $8 + 7 = 15$. Choice B (15) repeats the total instead of finding the missing addend. Choice C (6) is one less than the correct answer. Choice D (8) repeats the number already given in the equation.
11. Two numbers add up to 16. One of the numbers is 9. What is the difference between the two numbers?
    Answer: 2 The two numbers are 9 and 7, since 9 plus 7 equals 16. The difference between 9 and 7 is 2. Choosing 7 gives the other number itself, not the difference between the two numbers. Choosing 9 repeats the given number instead of finding the difference. Choosing 16 repeats the total sum instead of subtracting the two numbers.
12. Carlos has $6$ red marbles and $7$ blue marbles. He trades $4$ red marbles for $4$ green marbles. How many marbles does Carlos have in total now?
    Answer: $13$ marbles total 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.
13. A store has 7 red pens and 8 blue pens. How many pens are there in all?
    Answer: 15 7 red pens plus 8 blue pens is 15 pens total. A student might get 16 by rounding 7 up to 8 and doubling. A student might get 14 by only counting one type of pen twice. A student might get 13 by mixing up this fact with 8 + 5. Adding all the pens together, red and blue, gives 15.
14. Sofia has 9 stickers and gets 7 more. How many stickers does she have now?
    Answer: 16 Sofia has 16 stickers now, because $9 + 7 = 16$. Choice A (17) counts one sticker too many. Choice C (14) counts two stickers too few. Choice D (15) counts one sticker too few.
15. Maya has 9 stickers and gets 7 more. How many stickers now?
    Answer: 16 The correct answer is 16, because 9 + 7 = 16. Choice A (13) is too low for this sum. Choice B (14) is also too low, off by two. Choice D (15) is one sticker short of the correct total.
16. Calculate $8+5$.
    Answer: 13 Adding 8 and 5 gives 13. Choice B, 12, is off by one below the correct total. Choice C, 14, is off by one above the correct total. Choice D, 15, is off by two above the correct total.
17. $16 - ? = 8$. What number goes in the blank?
    Answer: 8 16 minus 8 equals 8, so 8 goes in the blank. 9 is close, but 16 minus 9 equals 7, not 8. 7 is close, but 16 minus 7 equals 9, not 8. 6 is too small; 16 minus 6 equals 10, not 8.
18. Sofia has 9 stickers and gets 7 more. How many stickers does she have now?
    Answer: 16 Sofia starts with 9 stickers and gets 7 more, so she has 9 plus 7, or 16 stickers. Choice B is 2 less than the correct total, a common counting error. Choice C is 1 less than the correct total. Choice D is 1 more than the correct total, from counting one sticker twice.
19. Lisa is thinking of a number. When she adds $8$ to her number, she gets $15$. When she adds $6$ to her number, what does she get?
    Answer: $13$ This is a "working backwards" problem where you need to find Lisa's mystery number first, then use it to answer the question. When Lisa adds $8$ to her number, she gets $15$. To find her original number, you work backwards by subtracting: $15 - 8 = 7$. So Lisa's number is $7$. You can check this: $7 + 8 = 15$ ✓ Now that you know Lisa's number is $7$, you can answer the actual question. When she adds $6$ to her number: $7 + 6 = 13$. Looking at the wrong answers: Choice A ($11$) might come from subtracting instead of adding: $17 - 6 = 11$, but this uses the wrong operations. Choice B ($23$) likely comes from adding $8 + 15 = 23$, which mixes up the given information instead of finding the original number. Choice C ($21$) probably comes from adding $15 + 6 = 21$, which incorrectly uses $15$ (the result) instead of $7$ (Lisa's actual number). The correct answer is D: $13$. Study tip: In "mystery number" problems, always find the unknown number first by working backwards, then use that number to solve what the question is actually asking. Don't get distracted by the numbers given in the problem—focus on finding the starting number first.
20. What is $15 - 8$?
    Answer: 7 $15 - 8 = 7$, so Choice C is correct. Choice A repeats one of the original numbers instead of subtracting. Choice B repeats the other original number. Choice D is one less than the correct difference.

Start this quiz