Rapid Addition and Subtraction

Addition and subtraction underpin almost every QR calculation. Whether you are finding a total from a table, calculating a difference, or working out a percentage change, you will add or subtract numbers. This lesson teaches three mental strategies — rounding and adjusting, left-to-right addition, and the complement method — that make these operations fast and reliable.

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Strategy 1: Rounding and Adjusting

The idea is simple: round one or both numbers to make the calculation easy, then adjust for the rounding.

Addition by Rounding

Method: Round one number up to the nearest convenient value, add, then subtract the amount you rounded.

Example: 387 + 245

1. Round 387 up to 400 (added 13)
2. 400 + 245 = 645
3. Adjust: 645 − 13 = 632

Example: 1,278 + 495

1. Round 495 up to 500 (added 5)
2. 1,278 + 500 = 1,778
3. Adjust: 1,778 − 5 = 1,773

When to use this: Whenever one number is close to a round number (ending in 7, 8, 9 or 1, 2, 3 relative to the nearest hundred or thousand).

Subtraction by Rounding