Fast Addition & Subtraction
Use round numbers, compensation and complements to simplify mental arithmetic.
Add 99247 + 99 = 346
247 + 100 = 347
347 − 1 = 346
347 − 1 = 346
Why: 99 is one less than 100.
Subtract 98500 − 98 = 402
500 − 100 = 400
400 + 2 = 402
400 + 2 = 402
Why: 98 is two less than 100.
Make a round number38 + 27 = 65
38 + 2 = 40
Move 2 from 27 → 25
40 + 25 = 65
Move 2 from 27 → 25
40 + 25 = 65
Why: Moving the same amount between addends keeps their sum unchanged.
Complements to 100100 − 67 = 33
67 + 3 = 70
+30 = 100
Total added = 33
+30 = 100
Total added = 33
Why: Subtraction can be viewed as finding the amount needed to reach the target.