This is an arithmetic progression, i.e. 16,18,20,22,24,26,28.......... where 16 is the number of people seats in 1st row, 18 seats in second row, 20 seats in third row and so on. The Total number of seats in 20 rows = n/2{2a+(n-1)d}. Here, d = 2 , n =20, a = 16. Thus, Sum = 10 {32+ 19*2} = 10 {70} = 700 seats in total.
