How much would it cost for a $100 skateboard with a 30% discount and 7.5% sales tax?

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

Sum is 17 and their difference is 3

Ineed with this problem 21 show steps