In a theater, the first row has 12 seats and each row has 2 more seats than previous row. If the last row has 48 seats, what is the total number of seats in the theater?
Choices.
A) 550 B) 570 C) 590 D) 610 E) 630

Rationale: "the number of seats in the theater is 12 + ( 12 + 2 ) + . . . + ( 12 + 36 ) = 19 ( 12 ) + 2 ( 1 + 2 + . . . + 18 ) = 19 ( 12 ) + 2 ( 18 ) ( 19 ) / 2 = 19 ( 12 + 18 ) = 19 ( 30 ) = 570 the answer is b ."
Answer: B