Finding Friendly Numbers

Mental Math · Axiom Academy

LESSON Finding Friendly Numbers Round numbers are easy to compute with — so take a short detour through the nearest friendly one, then compensate for the change you made. 1. Borrow to a Friendly Number, Then Repay Take 49 + 36 . The number 49 is awkward, but 50 is friendly. So round 49 up to 50 and add the easy way — just remember you nudged it up by 1. That 1 is a debt you pay back at the very end. Subtraction has a twist. For 52 − 19 , round the number you are taking away — 19 becomes a friendly 20. But taking away 20 removes 1 too much, so this time you add that 1 back. The compensation flips direction. 3. Even Friendlier: Detour Through 100 Tens are not the only friendly numbers — 100 is one of the friendliest of all, because it is a clean place-value unit. For 98 + 27 , notice that 98 is just 2 shy of a whole hundred. Complete the hundred, add, then hand back the 2. Fifties and twenty-fives are friendly too The same move works for any round number. For 76 + 48, round 48 up to a friendly 50: And 25 is friendly for money-style sums: 87 + 26 = 87 + 25 + 1 = 113. 4. When Is the Detour Worth It? This detour is a tool, not a law. It shines when a number sits right next to a friendly one, so the fix is tiny. When both numbers are stranded in the middle of a decade, the adjustment is as much work as the original problem — so just add. one number ends in 8, 9, 1, or 2 (like 49, 98, 31, 52) — it is a tiny hop to a friendly number, so the payback is 1 or 2.

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