Theater Seating

The Mathematics Theater has twenty-five seats in the first row, twenty-seven seats in the second row, twenty-nine seats in the third row, and so on. How many seats are in the theater if there are fifteen rows in all?

Extension:
If the theater had 15 rows with r seats in the first row and t seats in the 15th row, then how many seats are there all together?

(Source: Adapted from Mathematics Teaching in the Middle School, Sep-Oct 1996)