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 how it is being added.
Easy tens only
40 + 3020 + 5060 + 10 Mixed tens and ones
42 + 3523 + 1451 + 26 Numbers like 40 and 30 are just tens, but 42 has 4 tens plus 2 ones, so it is mixed.
Part B: Fill in the Blank
Write the missing word or number on each line.
1. 42 + 35: tens part is 40 + 30 = 70.
4 tens plus 3 tens is 7 tens, which equals 70.
2. 42 + 35: ones part is 2 + 5 = 7.
2 ones and 5 ones combine to make 7 ones.
3. Put the tens and ones back together: 70 + 7 = 77.
70 plus 7 ones equals 77, the final sum of 42 + 35.
4. Break apart 53 + 24. The tens part is 50 + 20 = 70.
Five tens plus two tens gives seven tens, which is 70.
5. Break apart 61 + 28. The full sum is 89.
60 + 20 is 80 and 1 + 8 is 9, so combined they make 89.
Part C: True or False?
Read each statement. Circle True or False.
1. 44 + 32 can be split into 40 + 30 and 4 + 2.
True False
44 is 40 + 4 and 32 is 30 + 2, so the split is correct.
2. To add 50 + 40, you need to break apart any ones.
True False
Both numbers end in zero, so there are no ones to break apart.
3. When you break apart 26 + 13, the ones add to 9.
True False
The ones are 6 and 3, and 6 + 3 equals 9, so the statement is true.
4. Breaking apart numbers makes mental math harder.
True False
Splitting into tens and ones gives smaller parts that are easier to add mentally.