Answer:
660
Explanation:
its going to be an arithmetic series so write out the first few terms of thr first row then second row etc
20, 24, 28, 32...
the formula for an arithemtic series is Un = a + d(n-1) where a is the first term and d is the common difference
in this sequence, a = 20 and d = 4
so the nth term formula for this sequence is
Un = 20 + 4(n-1) where n is the number of the row
The sum formula of an arithmetic series is
Sn = 0.5n (2a+d(n-1))
in other words, Sn is the sum of n terms. e.g. if you wanted to find the sum of the first 5 rows tou could sum in n=5 as well as the a and d values from above instead of doing n1+n2+n3+n4 etc
sub in a=20, d=4 and n=15
total seats of 15 rows = (0.5x15)(2x20 + 4+14)
= 7.5 (40+48)
= 660