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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.
The idea is simple: round one or both numbers to make the calculation easy, then adjust for the rounding.
Method: Round one number up to the nearest convenient value, add, then subtract the amount you rounded.
Example: 387 + 245
Example: 1,278 + 495
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).
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