Problem 1 · 2010 Math Kangaroo
Easy
Arithmetic & Operations
grouping
How much is 12 + 23 + 34 + 45 + 56 + 67 + 78 + 89?
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Answer: C — 404
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Hint 1 of 2
Pair the terms or just add carefully — the answer is one of the listed totals.
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Hint 2 of 2
Group into pairs that make the same round number before summing.
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Approach: direct addition with pairing
- Add the eight numbers: 12+23+34+45+56+67+78+89.
- Pairing from the ends (12+89, 23+78, 34+67, 45+56) gives 101 each, four times.
- 4×101 = 404.
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