Making Tens: Reorder to Build Friendly Numbers
Level 1 Mental addition · Reviewed by The NumThink editorial team · Last reviewed
Short version: Reorder the sum so friendly pairs combine first: 47 + 38 + 13 → 47 + 13 = 60, then 60 + 38 = 98.
This page is part of the NumThink Mental addition course. Read why the method works, watch it run on worked examples, then drill it in the practice box below until it feels automatic.
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No signup · instant feedbackWhy it works
Addition is commutative — order is yours to choose. Some pairs (7+3, 47+13, 126+74) land exactly on a ten or hundred, collapsing three hard steps into one trivial one.
Scanning for these pairs is a recognition habit, exactly like spotting ×25 or numbers near 100 in multiplication.
The steps
- Scan for pairs. 47 + 38 + 13: the units 7 + 3 make a ten — that pair is 47 + 13.
- Combine the pair. 47 + 13 = 60.
- Add what is left. 60 + 38 = 98.
Worked examples
Easy 8 + 7 + 2
Show the thinking
8 + 2 = 10
10 + 7 = 17.
Answer: 17
Medium 47 + 38 + 13
Show the thinking
47 + 13 = 60
60 + 38 = 98.
Answer: 98
Hard 126 + 54 + 74
Show the thinking
126 + 74 = 200
200 + 54 = 254.
Answer: 254
When to use it
- Any sum of three or more numbers — receipts, lists, score tallies.
- Pairs ending in complementary units (…3 + …7, …4 + …6, …1 + …9).
When NOT to use it
- Two-number sums — nothing to reorder.
- When no pair lands near a ten — forcing it adds work.
Common mistakes
Losing a number while reordering. Dropping the 38 after combining 47 + 13. Fix: Physically cross off pairs as you use them (on paper) or repeat the remaining list aloud.
Forcing bad pairs. Combining 47 + 38 = 85 first — a hard step that creates two medium steps. Fix: Ask one question: does this pair END in 0? If not, keep scanning.
Check yourself: After collapsing a pair the running total should be round (60, 200) — if it is not, you picked the wrong pair.