Help please !! will give brainliest:))

Can someone explain the elimination method to me for this problem?

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

Write a story that matches with the expression 42x-5 pls hurry