Multiple choice general knowledge math & puzzles

A man ate 100 bananas in five days, each day eating 6 more than the previous day. How many bananas did he eat on the first day?

  1. 8

  2. 11

  3. 13

  4. 21
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A Correct answer
Explanation

Let the first day be x. The sum of an arithmetic progression is (n/2)(2a + (n-1)d). Here, 5/2(2x + 4*6) = 100. This simplifies to 5(x + 12) = 100, so x + 12 = 20, which means x = 8. The man ate 8, 14, 20, 26, and 32 bananas respectively.

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