Look at option A. What is the sum in the first problem? How do you know?
The sum is 84. Use base-ten blocks to show 3 tens and 6 ones and 4 tens and 8 ones. Put the tens and ones together. Trade 10 ones for 1 ten. You get 8 tens and 4 ones.
What is the difference in the second problem in option A? How do you know?
The difference is 68. Use base-ten blocks to show 9 tens and 3 ones. Regroup 1 ten to make 10 ones. Take away 2 tens and 5 ones to get 68.
Look at option B. How can you use friendly numbers to find the sum?
Move the numbers around so that the friendly numbers are next to each other. Add the friendly numbers first.
Explain which numbers should be moved or grouped when you use friendly numbers to find the sum.
35 and 15 each end in 5, so these numbers make a ten. 26 ends in 6, and 14 ends in 4, so these numbers also make a ten. Tens are easy to add, so move the numbers like this:
What is the sum?
The sum is 90.
Look at option C. What is the sum in the first problem? How do you know?
The sum is 568. Start at 425 on a number line. Add 1 hundred to get 525, 4 tens to get 565, and 3 ones to get 568.
What is the difference in the second problem in option C? How do you know?
The difference is 186. Start at 823 on a number line. Subtract 6 hundreds to get 223, 3 tens to get 193, and 7 ones to get 186.
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