The number of seats in the theater's rows represent an ARITHMETIC SEQUENCE.
In an arithmetic sequence, the next term in the sequence is found by adding a constant (a set amount), called the common difference.
For the theater, there are...
17 seats in row 1
20 seats in row 2
23 seats in row 3
26 seats in row 4....
How many seats are there in row 15?
How do you know?