Question 823778
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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</pre>