Question

A page was missing from a 72 - page book. When Josh added up the remaining page numbers, the sum was 2521. Which page was missing?
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Answer 1

The numbers 1 to 72 add up to (1+72) x 72/2 = 73 x 36 = 2628
If we take away what Josh got we have 2628 - 2521 = 107
Pages in a book are odd on the first side and even on the second, so pages 11 and 12 are on the same sheet.
107 / 2 = 53.5 so the missing page had 53 on one side and 54 on the other side.
The remaining pages would have been numbered 1, 2, 3, ..., 51, 52, 55, 56, ... 71, 72