Mental Math: Addition Strategies

Show your work for each problem.

1.
Which strategy is best for ?
1. a)Make a ten:
2. b)Use a calculator
3. c)Break apart:
4. d)Count on fingers
Answer: Make a ten:
Make a ten is the best strategy here. Take from to make , then add the remaining to get .
2.
Use make a ten: What is ?
Answer: 15
. We took from to make , then added the remaining .
3.
Use doubles: What is ?
Answer: 14
is a doubles fact. Memorizing doubles helps with near-doubles like .
4.
Use near doubles: What is ?
Answer: 13
Using near doubles: , then add more because . So .
5.
Which pair of numbers makes a ten?
1. a) and
2. b) and
3. c) and
4. d) and
Answer: and
. The other pairs: , , .
6.
Use break apart to solve
Answer: 59
1. Break apart : What are the tens? 30
2. Break apart : What are the tens? 20
3. Add the tens: 50
4. Add the ones: 9
5. Combine: 59
7.
Use friendly numbers: What is ?
Answer: 72
. We rounded to , added, then subtracted the we added.
8.
Use friendly numbers to solve
Answer: 133
1. What friendly number is close to? 100
2. Add: 135
3. How much extra did you add? () 2
4. Subtract the extra: 133
9.
A movie is 95 minutes long. If there are 15 minutes of previews, what is the total time?
1. a)100 minutes
2. b)110 minutes
3. c)115 minutes
4. d)105 minutes
Answer: 110 minutes
minutes. Or: minutes.
10.
Add from left to right:
Answer: 77
1. Start with tens: 70
2. Now add ones: 7
3. Combine: 77
11.
You have 37 euros and earn 28 more. How much do you have now?
Answer: 65
euros.
12.
Use left-to-right to solve
Answer: 390
1. Add hundreds: 300
2. Add tens: 80
3. Running total: 380
4. Add ones: 10
5. Final: 390
13.
What is ? Use friendly numbers.
Answer: 455
. Round up to , add to get , subtract to adjust.
14.
In a game, you scored 78, 45, and 22 points in three rounds. What is your total?
1. a)140 points
2. b)145 points
3. c)135 points
4. d)155 points
Answer: 145 points
Strategy: (make 100!), then points.
15.
Chain addition: Solve using smart grouping
Answer: 97
1. Notice the two 25s. Add them first: 50
2. Now add : 97
16.
Calculate mentally:
Answer: 398
. Or think: double is , subtract (one from each ).