In a theater, the first row has 14 seats and each row has 3 more seats than previous row. If the last row has 50 seats, what is the total number of seats in the theater?	Choose one:
A) 404  B) 408  C) 412  D) 416  E) 420

Rationale: the number of seats in the theater is 14 + ( 14 + 3 ) + . . . + ( 14 + 36 ) = 13 ( 14 ) + 3 ( 1 + 2 + . . . + 12 ) = 13 ( 14 ) + 3 ( 12 ) ( 13 ) / 2 = 13 ( 14 + 18 ) = 13 ( 32 ) = 416 the answer is d .
Answer: D