In a theater, the first row has 15 seats and each row has 2 more seats than previous row. If the last row has 53 seats, what is the total number of seats in the theater? Choose the correct choice from the following answers:  A) 600  B) 640  C) 680  D) 720  E) 760
The number of seats in the theater is   15 + (15+2) + ...+ (15+38)=   20(15) + 2(1+2+...+19)=   20(15) + 2(19)(20)/2=   20(15+19) = 20(34) = 680   The answer is 680.
<result>C</result>
In a theater, the first row has 15 seats and each row has 2 more seats than previous row. If the last row has 53 seats, what is the total number of seats in the theater?