84 is 15% of what number

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

How many credit hours will a student have to take for the two tuition costs to be equal? round the answer to the nearest tenth of an hour.

For what values of x is x2 + 2x = 24 true?