A set of consecutive positive integers beginning with 1 is written on a whitboard. one is erased. the average of the remaining numbers is 35 7/17 . what number was erased?
hint ( 1+2+3...+(N-1)+N = N*(N-1)/2 )
The number erased was 7. The integers added were 1+2+...+69 (with the 7 missing). Incidentally, the formula for the sum of the first n numbers is actually n(n+1)/2 (not (n-1)).