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Missing Pages (Posted on 2009-11-13) Difficulty: 2 of 5
A chunk of pages is missing from a book. The sum of the missing page numbers is 9808. What are those page numbers?

See The Solution Submitted by Brian Smith    
Rating: 3.5000 (2 votes)

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Solution Solution | Comment 5 of 7 |

The pages numbered 291 through 322 are this missing pages.

As the sum of page numbers from 1 to n is a triangular number [that is, equal to n(n+1)/2], the sum of page numbers of the chunk of missing pages can be formulated, where X is the final missing page and Y is the first missing page, as:
X(X+1)/2 - Y(Y-1)/2  = 9808.

322(323)/2 - 291(290)/2  = 9808


  Posted by Dej Mar on 2009-11-13 17:37:58
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