Mental Math Within 100 — Answer Key
Part A: Sort the Words
Sort each word or number into the correct category box.
1. Sort each problem by whether you add 10 or add a bigger ten like 20 or 30.
Add 10
54 + 1041 + 1018 + 10 Add 20, 30, or more
27 + 2063 + 3035 + 40 Problems like 54 + 10 and 41 + 10 add one ten, while 27 + 20 and 63 + 30 add two or three tens.
Part B: Fill in the Blank
Write the missing word or number on each line.
1. 54 + 10 = 64 by raising the tens digit by one.
54 has 5 tens and 4 ones; adding one ten makes 6 tens and 4 ones, which is 64.
2. 27 + 20 = 47 by adding two tens in your head.
Two tens added to 27 goes 27, 37, 47, so the sum is 47.
3. 63 + 30 = 93 by adding three tens.
63, 73, 83, 93 shows three jumps of ten, so 63 + 30 equals 93.
4. Adding 10 to 41 changes only the tens digit, giving 51.
41 plus 10 is 51 because only the tens place goes up by 1.
5. 35 + 40 = 75 by jumping four tens up from 35.
35, 45, 55, 65, 75 shows four tens added, so 35 + 40 equals 75.
Part C: True or False?
Read each statement. Circle True or False.
1. When you add 10 to any number, only the tens digit changes.
True False
Adding 10 adds one to the tens place while the ones digit stays the same.
2. 27 + 20 equals 29 because you just add the 2.
True False
20 is two tens, so 27 + 20 is 47, not 29; the tens change, not the ones.
3. To add 35 + 40 mentally, you can count four tens up from 35.
True False
Four tens from 35 lands on 75, which equals 35 + 40 through mental count-on.
4. Adding a number that ends in zero is harder than adding ones.
True False
Numbers ending in zero only change the tens place, so they are usually easier than regrouping ones.