Use doubles practice answers:

1. Explain 2 ways to use doubles to help you figure out 7+8

• 7+7=14, and 8 is one more than 7, so 7+8=14+1=15
• 8+8=16, and 7 is one less than 8, so 7+8 is one less than 16, so it is 16-1=15

2. Explain 3 ways to use doubles to help you figure out 9+7

• 7+7=14 and 9 is 2 more than 7, so 7+9 is 2 more than 14, so 7+9=14+2=16
• 9+9=18 and 7 is 2 less than 9, so 7+9 is 2 less than 18, so 7+9=18-2=16
• 7 is one less than 8 and 9 is one more than 8, so if you give one from the 9 to the 7, you get 8+8, so 7+9=8+8=16

3. For each of these math fact problems, tell whether it is

• a double,
• a problem you can solve by a use doubles strategy or
• a fact not closely related to a double.

a. 5+7 you can solve this by a use doubles strategy (doubles+2, doubles -2 or sandwich doubles)

b. 8+8 this is a double

c. 9+4 this isn't closely related to a double because 9 is more than 2 more than 4

d. 6+5 you can solve this by a use doubles strategy (doubles +1 or doubles -1)

e. 8+4 this isn't closely related to a double because 8 is more than 2 more than 4. Some people get mixed up because 8 is double 4, but that doesn't really help you figure out 8+4 any more efficiently than you would otherwise.

4. Describe the facts for which a use doubles strategy is efficient

Addition facts where the addends have a difference of 1 or 2.

5. Write a word problem for introducing the use doubles to add strategy.

You should have written an addition word problem where addend numbers in your problem were different by only 1 (one number was 1 more than the other). If you wrote a PPW-WU problem, that's helpful too.

6. What do children need to be able to do before they can use a use doubles strategy effectively?

• They need to know a lot of the doubles
• They need to be able to recognize when one number is 1 or 2 greater than another number
• They need to be able to add and subtract 1 and 2 from numbers.