Question

in a lecture hall at a collage, the number of seats per row increased at a constand rate. The professor counts 21 seats in row 3 and 33 seats in row 7. How many sets are there in row 18?

Answer 1

66 seats

Explanation:

The rate of increase is ...

(change in seats)/(change in rows) = (33 -21)/(7 -3) = 12/4 = 3 . . . seats/row

Then row 18 will have (18-3)·3 = 45 more seats than row 3, hence ...

21 + 45 = 66 . . . . seats in row 18

_____

An explicit formula for the number of seats can be written as ...

s = 21 +3(n-3) = 12 +3n

For n=18, this is ...

s = 12 + 3(18) = 12 +54 = 66 . . . seats in row 18