the first row of an adutorium has 42 seats and each row after has 3 more than the row before it. How many seats are in row 25 and which row has 90 seats?

Algebra time.

We start with 42, this is our base number, we cannot go below this one. Since we add 3 chairs every row, then we add 3 (x) to every row after the first one. The first row has 42 chairs. The second has 42 + 3 (1), and the third row has 42 + 3 (2). X is going to be one less than the row number you're in. So for the 25th row we do 42 + 3 (24). And for the one with 90 seats we have 42 + 48 = 90. There's a difference of 48 chairs between 42 and 90. So we divide 48 by 3 to see the value of x. 48/3 is 16, so the equation is 42 + 3 (16) = 90. Therefore we're on the seventeenth row. An easier way is to start at 39 (42-3) and then do 39 + 3 (1) as the first row.