In a theater, there are 20 chairs in the first row, each row has 4 more chairs than the previous row, and there are 15 rows. How many chairs are there in the theater?
Use AP formula to sum them up:
Sum = 15/2 * [2*20 + 4*14 ]
Sum = 7.5 * [40 + 56 ]
Sum = 7.5 * 96
Sum = 720 chairs in the theater.
