Atheater has 20 rows of chairs. the first row has 16 chairs, the second has 18, the third has 20, and so on. in total, the theater seats people.
how many people does the theater seat in total?

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.