35. A person hired a firm to build a CB radio tower. The firm charges $100 for labor for
the first 10 feet. After that, the cost of the labor for each succeeding 10 feet is $25
more than the preceding 10 feet. That is, the next 10 feet will cost $125, the next
10 feet will cost $150, etc. How much will it cost to build a 90-foot tower?
cost = 100+ 125 + 150 + ... for 9 terms
looks like an arithmetic series to me
a = 100
d = 25
n = 9
apply the sum formula