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)
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
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