Multiply by Multiples of 10 - 3rd Grade Math

1. What is $8 \times 40$?
   Answer: 320 320 is correct because 8 times 40 equals 320. 80 is incorrect because it uses only one of the factors. 3200 is incorrect because it adds an extra zero, mistaking the place value. 48 is incorrect because it comes from adding 8 and 40 instead of multiplying them.
2. A farmer plants flowers in rows. She plants 5 rows with 60 flowers each, then plants 2 more rows with 60 flowers each. If she gives away 3 rows worth of flowers, how many flowers does she keep?
   Answer: $240$ flowers kept Total rows planted: $5 + 2 = 7$ rows. Total flowers: $7 \times 60 = 420$ flowers. Flowers given away: $3 \times 60 = 180$ flowers. Flowers kept: $420 - 180 = 240$ flowers. Choice A shows first planting only, choice B shows total planted, and choice D shows flowers given away.
3. Maria has 4 boxes of stickers. Each box contains 30 stickers. She gives away 2 boxes to her friends. How many stickers does Maria have left?
   Answer: $60$ stickers First, find total stickers: $4 \times 30 = 120$ stickers. Then find stickers given away: $2 \times 30 = 60$ stickers. Finally, subtract: $120 - 60 = 60$ stickers left. Choice B represents $3 \times 30$, choice C represents the original total, and choice D represents just one box.
4. A toy store has 8 shelves. Each shelf holds 70 toy cars. The store sells 3 shelves worth of toy cars. How many toy cars are still in the store?
   Answer: 350 toy cars The store started with 8 times 70, which is 560 toy cars, and after selling 3 shelves worth, which is 3 times 70, or 210 cars, 560 minus 210 leaves 350 toy cars. Choice B (210 toy cars) is the number of cars sold, not the number remaining. Choice C (560 toy cars) is the total before any cars were sold. Choice D (490 toy cars) comes from subtracting only one shelf's worth of cars instead of three.
5. Mia buys 6 packs of 30 stickers. How many stickers total?
   Answer: 180 stickers 6 packs of 30 stickers is 6 x 30 = 180 stickers, making Choice B correct. Choice C (1800) adds an extra zero to the correct product. Choice A (60) and Choice D (18) both come from using the wrong number of packs or stickers per pack.
6. Mina has 5 boxes with 80 crayons in each box. How many crayons does she have in total?
   Answer: 400 crayons 400 crayons is correct because $5 \times 80 = 400$. 4000 crayons is incorrect; it has an extra zero, far more than the actual product. 85 crayons is incorrect because it comes from adding 5 and 80 instead of multiplying. 40 crayons is incorrect; it's half of the correct product, not the actual total.
7. Use the pattern: If $9\times7=63$, what is $9\times70$?
   Answer: 630 Since $9 \times 7 = 63$, multiplying by 70 instead of 7 makes the product ten times larger, which is 630. Choice A (90) comes from adding instead of using the pattern correctly. Choice C (63) repeats the original fact without adjusting for the extra ten. Choice D (6300) makes the product one hundred times larger instead of ten times larger.
8. Use the pattern: If $4\times5=20$, what is $4\times50$?
   Answer: 200 Since 50 is 10 times 5, the product 4 times 50 is 10 times the product of 4 times 5, so 4 times 50 equals 200. Choice A, 20, repeats the original fact without scaling it up. Choice C, 40, only doubles the original product instead of multiplying it by 10. Choice D, 2000, scales the product by 100 instead of 10.
9. What is $7 \times 30$?
   Answer: 210 Multiplying 7 by 30 gives 210, since $7 \times 3 = 21$ and then multiplying by 10 gives 210. Choice A (21) is the result of $7 \times 3$ without accounting for the extra ten. Choice C (70) mixes up the factors, treating it like $7 \times 10$. Choice D (37) comes from adding 7 and 30 instead of multiplying.
10. A craft store has 200 beads. Sarah uses 100 of them to make bracelets. How many beads does she have left?
    Answer: 100 beads 100 beads is correct because $200 - 100 = 100$. 50 beads is incorrect; it doesn't match subtracting 100 from 200. 200 beads is incorrect because it ignores that beads were used at all. 150 beads is incorrect; it doesn't correspond to subtracting the full 100 beads used.
11. Use the pattern: If $4\times5=20$, what is $4\times50$?
    Answer: 200 This question tests multiplying one-digit numbers by multiples of 10 in the range 10-90 (CCSS.3.NBT.3), specifically using place value strategies and properties of operations. To multiply a digit by a multiple of 10, use the pattern: first multiply the digit by the unit digit, then multiply the result by 10. For example, if 4×5=20, then 4×50 means multiply 20 by 10 to get 200. Another way: 4×50 means 4 groups of 50, which is the same as 4 groups of 5 tens = 20 tens = 200. In this problem, students use the pattern: if 4×5=20, what is 4×50? This represents extending a basic fact to multiply by a multiple of 10. Choice A is correct because 4×50=200 using the pattern (4×5=20, then ×10=200) or place value (4×5 tens = 20 tens = 200). This demonstrates understanding of multiplying by multiples of 10. Choice B is incorrect because it shows only 4×5=20 without multiplying by 10. This error occurs when students don't understand the relationship between basic facts and multiples of 10. To help students multiply by multiples of 10: Connect to basic facts (if you know 4×5=20, then 4×50=200). Use place value language (4×50 = 4×5 tens = 20 tens = 200). Practice skip counting by 10s, 20s, 30s, etc.
12. Which product is GREATER than $5 \times 80$?
    Answer: $7 \times 60$ When you're comparing products without a calculator, the fastest strategy is to compute the target value first, then check each option against it. Here, $5 \times 80 = 400$, so you're looking for a product greater than 400. Now test each choice: Option A: $7 \times 60 = 420$. That's greater than 400. ✓ Option B: $4 \times 90 = 360$. Less than 400. Option C: $6 \times 60 = 360$. Less than 400. Option D: $8 \times 40 = 320$. Less than 400. Only A produces a product larger than $5 \times 80$, making it the correct choice. Notice a helpful pattern here: just because a product has "bigger looking" numbers doesn't mean the result is bigger. For example, D has an 8 (larger than the 5 in the original), but because the other factor dropped from 80 to 40, the total shrank. Multiplication depends on both factors working together, not just one being large. Study tip: When comparing multiplication expressions, always calculate the target value first and write it down. Then work through each option in order. Also, remember that multiplying by a multiple of 10 is quick — just multiply the non-zero digits and add the zero back on (for example, $7 \times 6 = 42$, so $7 \times 60 = 420$). This shortcut will save you time on many 3rd-grade multiplication problems.
13. Kai has 6 groups of 20 marbles. How many marbles in all?
    Answer: 120 marbles Multiplying 6 groups by 20 marbles each gives 6 times 20, which is 120 marbles. Choice A, 12 marbles, comes from multiplying 6 by 2 and dropping a zero. Choice B, 60 marbles, comes from multiplying 6 by 10 instead of 20. Choice D, 26 marbles, comes from adding 6 and 20 instead of multiplying.
14. Ava has 7 bags with 10 marbles each. How many marbles does she have in all?
    Answer: 70 marbles 7 bags with 10 marbles each gives 7 times 10, which equals 70 marbles, so A is correct. Choice B (17) adds the numbers instead of multiplying. Choice C (700) adds an extra zero to the correct product. Choice D (7) repeats the number of bags instead of finding the total.
15. Jamal saves $30 each week; after 8 weeks, how much saved?
    Answer: $240 This question tests multiplying one-digit numbers by multiples of 10 in the range 10-90 (CCSS.3.NBT.3), specifically using place value strategies and properties of operations. To multiply a digit by a multiple of 10, use the pattern: first multiply the digit by the unit digit, then multiply the result by 10. For example, 6×30: Think of 30 as 3 tens, multiply 6×3=18, then multiply by 10 to get 180 (or think: 18 tens = 180); another way: 6×30 means 6 groups of 30, which is the same as 6 groups of 3 tens = 18 tens = 180; the pattern is: if 6×3=18, then 6×30=180 (one more zero because 30 has one zero). In this problem, Jamal saves $30 each week for 8 weeks. This represents the multiplication 8×30. Choice B is correct because 8×30=240 using the pattern (8×3=24, then ×10=240) or place value (8×3 tens = 24 tens = 240). This demonstrates understanding of multiplying by multiples of 10. Choice C is incorrect because it only multiplies 8×3=24 and forgets to multiply by 10. This error occurs when students don't complete the pattern. To help students multiply by multiples of 10: Connect to basic facts (if you know 7×4=28, then 7×40=280); use place value language (7×40 = 7×4 tens = 28 tens = 280); model with base-10 blocks (7 groups of 4 tens rods); practice skip counting by 10s, 20s, 30s, etc.; show pattern with arrays (7 rows of 40 objects arranged as 4 tens per row); teach: multiply the digits, then add one zero (because 10 has one zero, 40 has one zero, etc.); watch for students who forget to multiply by 10 or add too many zeros.
16. Tessa is packing gift bags. She fills 8 bags with 30 stickers each. Then she realizes she also needs 6 bags with 20 stickers each. How many stickers does Tessa need in all?
    Answer: $360$ When a word problem describes two separate groups being combined, your plan should be: find the total of each group first, then add those totals together. Watch for the word "also" — it's a clue that you need to combine, not compare. Here, Tessa fills 8 bags with 30 stickers each, so that group needs $8 \times 30 = 240$ stickers. Then she needs 6 more bags with 20 stickers each, which is $6 \times 20 = 120$ stickers. Adding both groups: $240 + 120 = 360$ stickers in all, which matches B. Choice A (240) is only the stickers for the first set of bags — you'd get this if you forgot to add the second group. Choice C (280) comes from a mix-up like $8 \times 20 + 6 \times 20 = 160 + 120$, using the wrong number of stickers for the first group. Choice D (420) results from swapping the stickers-per-bag between the two groups: $8 \times 30 + 6 \times 30$ or $8 \times 20 \times \text{something}$ — essentially applying the wrong sticker count and over-counting. A helpful strategy: underline each pair of numbers that "go together" in the problem (8 bags with 30, 6 bags with 20) before multiplying. This keeps you from accidentally pairing the wrong numbers. And always ask yourself, "Am I being asked for a part or the whole?" Here, "in all" signals the whole, so you must add both products.
17. Which statement is TRUE?
    Answer: $8 \times 70 = 560$ When you multiply a whole number by a multiple of 10, there's a shortcut: multiply the non-zero digits together, then attach the zero(s) from the multiple of 10 to the end of your answer. For $8 \times 70$, first think of 70 as $7 \times 10$. Multiply $8 \times 7 = 56$, then attach the one zero from the 70, giving you $560$. That matches choice D. Now let's look at the traps. Choice A ($56$) forgets to attach the zero at the end — the student multiplied $8 \times 7$ correctly but ignored that the 7 was actually 70, so the answer is 10 times too small. Choice B ($480$) comes from a multiplication error: it looks like the student multiplied $8 \times 6 = 48$ instead of $8 \times 7 = 56$, then added a zero. Always double-check your basic facts. Choice C ($5{,}600$) attaches two zeros instead of one — this would be the correct answer for $8 \times 700$, not $8 \times 70$. Be careful to count exactly how many zeros are in the original number. Study tip: When multiplying by multiples of 10, count the zeros in the problem and make sure the same number of zeros appears in your answer. For $8 \times 70$, there's exactly one zero in the problem, so your answer must end in exactly one zero.
18. Compare: $4 \times 70$ ___ $6 \times 50$
    Answer: $<$ When you see a comparison problem like this, don't try to guess — just calculate both sides and then compare them. Multiplying by multiples of 10 is quick if you use a shortcut: multiply the non-zero digits first, then attach the zero at the end. For the left side, $4 \times 70$: think $4 \times 7 = 28$, then add the zero to get $280$. For the right side, $6 \times 50$: think $6 \times 5 = 30$, then add the zero to get $300$. Now compare: $280$ is less than $300$, so $4 \times 70 < 6 \times 50$. That makes B the correct choice. Choice A ($>$) would only be right if the left side were larger, but $280$ is smaller than $300$. Choice C ($=$) is a trap for students who might assume that since both problems use "small" numbers times "multiples of ten," they should come out the same — but the actual products are different. Choice D ("Cannot be determined") is never correct when you have exact numbers to multiply; you can always find the answer with arithmetic. Tip: When comparing two multiplication expressions, always solve each side completely before choosing a symbol. And remember the "multiples of 10" trick: multiply the basic facts, then tack on the zeros. It saves time and prevents careless mistakes.
19. A school cafeteria serves lunch to students in groups. If 7 groups of 40 students each eat lunch, and then 3 more groups of 40 students each eat lunch, what is the total number of students who ate lunch?
    Answer: $400$ students total First group: $7 \times 40 = 280$ students. Second group: $3 \times 40 = 120$ students. Total: $280 + 120 = 400$ students. Choice A shows only the first group, choice C shows $8 \times 40$, and choice D shows only the second group.
20. What is $4 \times 70$?
    Answer: 280 280 is correct because $4 \times 70 = 280$. 28 is incorrect; it's the product of 4 and 7 without accounting for the extra zero in 70. 2800 is incorrect because it has one too many zeros. 74 is incorrect; it doesn't represent multiplying 4 by 70.

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