The average of the integers N + 1, N + 2, ..., 2N is \(20\). What is N?
There are N numbers in the sum (N+1) + (N + 2) + ... + (2N)
Their sum is: N( (N + 1) + 2N ) / 2 = N(3N + 1) / 2
The average is: [ N(3N + 1) / 2 ] / N = (3N + 1) / 2
Since this average is 20: (3N + 1) / 2 = 20 ---> 3N + 1 = 40
3N = 39
N = 13
