Question

There are 21 rows of seats in an amphitheatre. Each row has 4 seats more than the row in front. The last row has 100 seats. How many seats are there altogether in the amphitheatre

Answer 1

this is just an A.P. with
d = 4
a = 100-20*4 = 20
So, the sum of 21 rows is

S = 21/2 (20+100)