Simple average question

Hi,
if a question says:
If the average of the first 16 positive integers is subtracted from the average next 16 positive integers, what is the result?

How come the average of each series is computed as:
(1 + 16)/2 and (17 + 32)/2 not (1+2+3...+16)/16 and (17+18+...+32)/16?

Thanks!