Topic 4 of 8
Adding and subtracting within 100
Step 4 · Place value to 1,000 topics
Add and subtract two-digit numbers using tens and ones, making or breaking a ten when you need to, and solve one- and two-step word problems.
Adding with a new ten
To add two-digit numbers, add the tens, add the ones, then put them together. 34 + 25: 30 + 20 = 50 and 4 + 5 = 9, so 59.
If the ones make 10 or more, they make a new ten. 47 + 38: tens 40 + 30 = 70; ones 7 + 8 = 15; 70 + 15 = 85.
Another way: jump to a ten first. 47 + 3 = 50, and 38 is 3 + 35, so 50 + 35 = 85.
Subtracting by breaking a ten
To subtract, take away the tens and the ones. 68 − 23: 60 − 20 = 40 and 8 − 3 = 5, so 45.
If there are not enough ones, break a ten into 10 ones. 52 − 17: 2 ones is not enough to take 7. Break one ten: 4 tens and 12 ones. 12 − 7 = 5 and 40 − 10 = 30. So 35.
Example: Breaking a ten
Work out 52 − 17.
- 52 is 5 tens and 2 ones; break a ten to make 4 tens and 12 ones.
- Ones: 12 − 7 = 5.
- Tens: 4 tens − 1 ten = 3 tens.
Answer35
Or count up: from 17 to 20 is 3, and from 20 to 52 is 32. 3 + 32 = 35.
Two-step word problems
Some problems take two steps. Do one step, then use its answer in the next.
Example: Two steps
A bus has 34 people. At a stop, 12 get off and 9 get on. How many are on the bus now?
- Step 1: 34 − 12 = 22.
- Step 2: 22 + 9 = 31.
Answer31 people
Check a subtraction by adding: 52 − 17 = 35 because 35 + 17 = 52.
Try it yourself
Work each one on paper first, then open the answer to check.
36 + 42
Show answer
78.
58 + 27
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85: 50 + 20 = 70, 8 + 7 = 15, 70 + 15 = 85.
75 − 31
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44.
63 − 28
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35: count up 2 to 30, then 33 to 63.
A box has 48 crayons. 19 are broken. How many are not broken?
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29 crayons, because 48 − 19 = 29.
Ana had 25 cards, won 18 more and gave 10 away. How many does she have?
Show answer
33: 25 + 18 = 43, then 43 − 10 = 33.