If each term of the sequence (a1, a2…, an)is either 7 or 77, and the sum of the sequence is 350, which of following could be the

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问题 If each term of the sequence (a₁, a₂…, a_n)is either 7 or 77, and the sum of the sequence is 350, which of following could be the value of n？

选项 A、45
B、42
C、40
D、39
E、36

答案C

解析假设在数列中，77一共出现m次，那么根据题意：77m+7×(n-m)=350，化简后得到：n+10m=50，五个选项中哪个能够为n呢?n=50-10m，必然为10的倍数，所以选择40。

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