Pages of a book
Number Theory
Level
2
The pages of a magazine are numbered starting from 1. Jack was asked to add up all the page numbers, but he included one page twice and got the sum as 130. What was the page number that Jack included twice?
The answer is 10.
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1 solution
Just use the formula (n/2)(n+1) and try it on the numbers you want.
I tried it on 10 and 20 first and then lastly on 15. Here's how,
(15/2)(15+1) = 120.
the difference between 130 and 120 is 10 which is the answer.