Question 176989
There are 20 seats in the first row and 20 + 50 - 1 = 69 seats in the last row.  You need the sum of the integers from 20 to 69 inclusive.


20 plus 69 = 89


21 plus 68 = 89


and so on


You have 25 (50 divided by 2) such pairs that add to 89, so 25 X 89 = 2225


In general, the sum of a series of integers is {{{((a + l)n)/2}}} where a is the first integer, l is the last integer, and n is the number of integers.