Quick Addition Summary

Mental Math · Axiom Academy

SUMMARY Quick Addition Summary Let's review the techniques that turn awkward addition into quick, confident calculation. Two numbers that complete each other to a ten or a hundred turn an awkward sum into a clean round one — 8+7=10+5=15 by making a ten, and 37+63=100 by pairing digits. Adding left to right, biggest place first, lets you hold just one running total and know the answer's rough size immediately — 34+33=60+7=67 . Compensation nudges one addend to a friendly number, adds the easy way, then undoes the exact same nudge on the total — 47+29=47+30-1=76 . Breaking apart addends keeps one number whole and feeds the other in as bites, choosing a bite that bridges through a round waypoint — 56+37=56+30+7=93 . With three or more numbers, scan first and pair complements wherever they hide, to any friendly subtotal — reordering never changes the sum: 12+5+8+15+10=50 . Casting out nines shrinks any number to a single check digit in seconds, catching most arithmetic slips before you re-add anything — though a match is only strong evidence, never proof. Core Concept Complements to 10 Every digit under ten has a partner that completes it to a ten — 1 & 9, 2 & 8, 3 & 7, 4 & 6, 5 & 5 — each pair sitting the same distance either side of five. Spot the pair and an awkward sum turns into a clean ten in one move: to add 8+7 , split the 7 into 2+5 , snap the 2 onto the 8 to make a ten, and the leftover 5 gives 8+7=10+5=15 .

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