Since each row as 8 chairs and they can add 3 chairs to each row,
the rows can now have 11 chairs in them.
they needed 5x as many chairs as 8x7. this equals 280 chairs (8x7=56--> 56x 5 = 280) 280 divided by 11 chairs per row is 25.48 which is 26 rows with some empty chairs left over, therefore your answer is 26 rows.