Question

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?

Answer 1

lets look:90 ft will be 9 sections,

100+25+50+75+ 100+ ....25(i)...where i goes from 1 to 8
cost= 100+ Sum25i from i=1 to INT(h-10/10 (h-80)

but this is an arithmetic progression

cost= 100+ n/2(2*25+(n-1)25)
= 100 (4(50+375))=100+425*4=1800

Answer 2

lets look:90 ft will be 9 sections,

100+25+50+75+ 100+ ....25(i)...where i goes from 1 to 8
cost= 100+ Sum25i from i=1 to INT(h-10/10 (h-80)

but this is an arithmetic progression

cost= 100+ n/2(2*25+(n-1)25)
= 100 (4(50+375))=100+425*4=1800