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What is 20+(5 x 2/5 )+ 3 show your work

For the rule of 78, for a 12-month period, the last term in the sequence is 12 and the series sums to 78. for an 10 month period, the last term is and the series sum is . for a 15 month period, the last term is and the series sum is . for a 20 month period, the last term is and the series sum is

[10 points, algebra 2]according to my answer key the restrictions are -1 and 4. but why not positive 1?