Adding Strings of Numbers

Mental Math · Axiom Academy

LESSON Adding Strings of Numbers Reorder before you add — scan the list, snap the complements into tens, and a long string collapses into a few round numbers. 1. Scan First, Then Pair to Ten Take 7 + 4 + 3 + 6 + 8 . Don't start adding — scan . The 7 and the 3 make ten; the 4 and the 6 make ten. Snap those two pairs together and you're left with 10 + 10 + 8 . The lone 8 just gets swept up at the end: 28 . Two easy tens and a leftover beat adding five numbers one at a time. Pair the complements, then sweep up the rest Same move on 9 + 6 + 1 + 4 + 5 : the 9 pairs with the 1, the 6 pairs with the 4, and the 5 rides along — 10 + 10 + 5 = 25 . 2. Friendly Subtotals — Not Just Tens Complements don't have to make ten. In 12 + 5 + 8 + 15 + 10 , the 12 and 8 snap to 20 , the 5 and 15 snap to 20 , and the 10 is already round. That's 20 + 20 + 10 = 50 — three round numbers instead of five awkward ones. Reach for whatever friendly total is closest: a ten, a twenty, a fifty. For 3 + 17 + 7 + 13 + 5 , two twenties are hiding: 3 + 17 and 7 + 13. That leaves 20 + 20 + 5 = 45 . 3. Long Strings Collapse — and It's Still Just Adding The longer the string, the more this pays off. 8 + 5 + 2 + 7 + 3 + 5 looks like a mouthful, but scan it and three pairs fall out: 8 + 2, 7 + 3, and 5 + 5 — three tens , so the answer is simply 30 . And because order never changes a sum, it's the exact same 30 you'd reach grinding straight through — just found far faster.

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