math
posted by lisa .
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 90foot tower?

cost = 100 + 125 + 150 + 175 + for 9 terms
This is an arithmetic series
a = 100
d = 25
n = 9
Sum(9) = (9/2)(2(100) + (91)(25))
= (9/2)(200 + 200) = 1800