theater seating
The Mathematics Theater has twentyfive seats in the first row, twentyseven seats in the second row, twentynine seats in the third row, and so on. How many seats are in the theater if there are fifteen rows in all?

Extensions 
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, SepOct 1996) 
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