The sum of the first four terms of an arithmetic progression is 56. The sum of last four terms of same progression is 112. If the first term is 11, find the number of terms in this arithmetic progression.
listfor(n, 0, 10, (11+2*n)) = (11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31) = 11 terms
