Mental Math: Multiplication Strategies

Show your work for each problem.

1.
Which strategy would be BEST for calculating ?
1. a)Double and halve
2. b)Multiply by 10, then divide by 2
3. c)Multiply by 9, then add 16
4. d)Break apart 16 into 10 + 6
Answer: Multiply by 10, then divide by 2
The multiply-by-5 trick is perfect here: , then .
2.
Use the break apart strategy: What is ?
Answer: 52
3.
Use the multiply-by-9 trick: What is ?
Answer: 72
4.
Calculate using the multiply-by-5 trick.
Answer: 70
1. First, multiply 14 by 10. What do you get? 140
2. Now divide 140 by 2. What is your answer? 70
5.
What is using the multiply-by-9 trick?
1. a)60
2. b)54
3. c)56
4. d)48
Answer: 54
6.
Use the break apart strategy to calculate .
Answer: 102
1. Break 17 into two parts: 10 + ? 7
2. Calculate 60
3. Calculate 42
4. Add the two results: 102
7.
Use doubling and halving: What is ?
Answer: 200
. By doubling 25 and halving 8, we get an easier calculation.
8.
A student calculated as follows: . What did they forget?
1. a)To subtract 15
2. b)Nothing, the answer is correct
3. c)To also calculate and add it
4. d)To divide by 2
Answer: To also calculate and add it
When breaking apart 15 into 10 + 5, you must multiply BOTH parts by 4: .
9.
Calculate using the multiply-by-5 trick.
Answer: 110
10.
Use doubling and halving to calculate .
Answer: 180
1. Double 15. What do you get? 30
2. Halve 12. What do you get? 6
3. Now calculate 180
11.
Using the break apart strategy, what is ?
1. a)86
2. b)88
3. c)92
4. d)96
Answer: 92
12.
Use the near squares strategy to calculate .
Answer: 899
1. Both numbers are close to 30. What is ? 900
2. 31 is 30 + 1, and 29 is 30 - 1. Using , what is ? 1
3. Subtract: 899
13.
A book costs 19 dollars. How much do 8 books cost? Use mental math.
Answer: 152
dollars.
14.
Calculate using the double and halve strategy.
Answer: 140
. Doubling 35 and halving 4 makes the calculation easy!
15.
A recipe needs 45 grams of sugar. You're making 6 batches. How many grams total?
Answer: 270
1. Break 45 into 40 + 5. What is ? 240
2. What is ? 30
3. Add the results: 270
16.
Which calculation shows the correct use of doubling and halving for ?
1. a)
2. b)
3. c)
4. d)
Answer:
Double 50 to get 100, halve 14 to get 7: . The other options either don't keep the product the same or aren't using the strategy correctly.
17.
What is using the multiply-by-5 trick?
Answer: 60
18.
Calculate using the multiply-by-9 trick.
Answer: 108
1. First, multiply 12 by 10. What do you get? 120
2. Now subtract 12 from 120. What is your answer? 108