Compatible Numbers

Show your work for each problem.

1.
Which number is compatible with to make ?
1. a)83
2. b)73
3. c)63
4. d)77
Answer: 73
. The ones: (carry 1). The tens: . So is compatible with .
2.
What number goes with to make ?
Answer: 55
. Ones: . Tens: .
3.
Which pair of numbers are compatible for making ?
1. a) and
2. b) and
3. c) and
4. d) and
Answer: and
. The other pairs: , , . Only and make exactly .
4.
What number goes with to make ?
Answer: 650
. Think of it as , then multiply by .
5.
Which compatible number would you use to estimate ?
1. a)
2. b)
3. c)
4. d)
Answer:
is very close to and is easy to multiply: . The actual answer is .
6.
Use compatible numbers to estimate . What is your estimate?
Answer: 250
, and . The actual answer is , so our estimate is very close!
7.
Find the compatible number for to make .
Answer: 38
1. What do you need to add to (ones digit) to get ? 8
2. You'll carry to the tens. What do you add to to get ? 3
3. Combine the digits: what is the compatible number? 38
8.
To estimate , which compatible number for would work best?
1. a)
2. b)
3. c)
4. d)
Answer:
. It's close to and divides evenly. The actual answer is .
9.
Estimate using compatible numbers, then find how close your estimate is to the actual answer.
Answer: 8
1. What compatible number is close to and easy to multiply? 30
2. Calculate: 120
3. Calculate the actual: 112
4. What is the difference between estimate and actual? 8
10.
What number goes with to make ?
Answer: 16
. Ones: (carry 1). Tens: .
11.
You want to estimate . Which compatible numbers would give the best estimate?
1. a)
2. b)
3. c)
4. d)
Answer:
and . So . The actual answer is , very close!
12.
A store sells books for 47 dollars each. Estimate the cost of 8 books using compatible numbers.
Answer: 400
1. What compatible number is close to 47 and easy to multiply? 50
2. Calculate the estimate: 400
3. Is your estimate higher or lower than the actual cost? higher
13.
Estimate using compatible numbers. What is your estimate?
Answer: 40
is close to and . The actual answer is , so our estimate is very accurate!
14.
You have 100 dollars and want to buy items costing 28 dollars, 37 dollars, and 19 dollars. Use compatible numbers to estimate if you have enough money.
Answer: 10
1. Round 28 dollars to the nearest ten: 30
2. Round 37 dollars to the nearest ten: 40
3. Round 19 dollars to the nearest ten: 20
4. Add the estimates: 90
5. How much would be left from 100 dollars? 10
15.
Which estimation uses compatible numbers correctly for ?
1. a)
2. b)
3. c)
4. d)
Answer:
is the best estimate. and . Actual: . Our estimate of is close and easy to calculate!
16.
What number goes with to make ?
Answer: 275
. Ones: . Tens: . Hundreds: .
17.
A baker needs to divide 453 cookies equally among 9 classes. Estimate how many cookies each class gets.
Answer: 50
1. What number close to 453 is divisible by 9? 450
2. Calculate: 50
3. Is this estimate slightly low or slightly high? low