Three-digit adding and taking away · Usually met around P2
Making 100
Fill up in steps: to the next 10, then 100.
How to do it
Method
Fill up in steps: to the next 10, then 100
63 → 70 → 100: 7 + 30 = 37
Another way
Column subtraction
100 − 63 = 37
In full
63 + ? = 100: fill it up in steps. First go to the next ten: 63 → 70 is 7. Then 70 → 100 is 30. 7 + 30 = 37. A whole ten goes straight up: 40 + ? = 100 → 60. When the ones aren't 0, check your answer: the ones make 10 and the tens make 9 (3 + 7 = 10, 6 + 3 = 9). You can also write it as 100 − 63 and use column subtraction with a borrow.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
30 + ? = 100
Show the answer and working
Answer: 30 + 70 = 100
30 + ? = 100: a whole ten goes straight up, 30 + 70 = 100
Example 2
73 + ? = 100
Show the answer and working
Answer: 73 + 27 = 100
73 + ? = 100: fill it up in steps, 73 → 80 is 7, 80 → 100 is 20, 7 + 20 = 27
Example 3
24 + ? = 100
Show the answer and working
Answer: 24 + 76 = 100
24 + ? = 100: fill it up in steps, 24 → 30 is 6, 30 → 100 is 70, 6 + 70 = 76
Practise it in the game →Three-digit adding and taking away · Free to start, unlimited questions, no download