An auditorium has 49 rows of seats. The first row contains 40 seats. As you move to the rear of the auditorium, each row has 3 more seats than the previous row.

Question

-How many seats are in the auditorium?

Answer 1

This is an arithmetic series
Sum(n) = (n/2)[2a + (n-1)d]
where a is the first term
n is the number of terms
d is the common difference, and
Sum(n) is the sum of the n terms

So you want Sum(49)
with a=40, d=-3 and n = 49

substitute