The right side of the orchestra section of the Nederlander Theater in New York City has 19 rows, and the last row has 27 seats. The number of seats in each row increases by 1 as you move toward the back of the section. How many seats are in this section of the theater.

The rows form an arithmetic sequence: each row has one more seat than the row in front of it. With 19 rows and 27 seats in the last row, the first row has 27 − 18 = 9 seats. The total is (9 + 27) × 19 ÷ 2 = 342 seats, so the correct choice is 342 seats.

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