The first 40 rows of seating in a certain section of a stadium have 25 seats, 28 seats, 31 seats, and so on. The 41st through the 75th rows each contain 125 seats. Find the total number of seats in this stadium section.

Answer 1

For terms from 1 to 40 you have an arithmetic series, where
a = 25, d=2 and n = 40
sum of those seats = (40/2)(2(25) + 39(2) )
= 2560
then there are 35 rows of 125 seats each or 4375 seats

so we have 6935 seats