1968 AMC 12 Problem 26

Attempt Problem 26 of the 1968 AMC 12 below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 1968 AMC 12 solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

26.

Let S=2+4+6+⋯+2N,S=2+4+6+\cdots+2N, where NN is the smallest positive integer such that S>1,000,000.S\gt1{,}000{,}000. Then the sum of the digits of NN is:
2727
1212
66
22
11
Answer: E
Concepts:arithmetic sequenceinequalitydigits
Small hint:
The sum is N(N+1)N(N+1)
Big hint:
Compare 999⋅1000999\cdot1000 and 1000⋅10011000\cdot1001 with one million
Solution:
We have S=N(N+1).S=N(N+1). For N=999,N=999, this is 999,000<1,000,000,999{,}000\lt1{,}000{,}000, while for N=1000N=1000 it is 1,001,000.1{,}001{,}000. Thus N=1000,N=1000, whose digit sum is 1.1.
Therefore, the correct answer is E.
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