Aset of numbers that follow a pattern, with a specific first number

(98 points) i need with 5 questions. answer definitions are welcomed, but not necessary.

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

Apositive number a or the same number a increased by 50% and then decreased by 50% of the result?