Answer: There will be 77 seats in the 13th row.
In the first row, there are 5 seats. To get the next row, we simply add on 6 to the current row.
If we want to have 12 additional rows added, he have to add on 12 groups of 6 chairs. So we multiply 12 by 6 and add on the original 5 chairs.
12 x 6 + 5 = 77 chairs
