That is solved for sum = 1620, not 1720 here: http://www.algebra.com/cgi-bin/jump-to-question.mpl?question=495769 There is no solution for 1720. That is a typo, the 1720 must be 1620. If you try to use 1720, the answer comes out with n as a fraction and the number of terms cannot be a fraction. Edwin