PSLE
In a school hall, chairs were arranged in rows such that there were exactly 24 chairs in each row. For a concert, Xavier removed 6 chairs from the school hall and rearranged all the chairs. There are now exactly 15 chairs in each row and 11 more rows than before. How many chairs are there in the school hall for the concert in the end?

	Before	After
Number	1 u	1 u + 11
Value	24	15
Total value	24 u - 6	15 u + 165

Number of rows at first = 1 u
Number of rows in the end = 1 u + 11

Number of chairs at first after the 6 chairs were removed
= 24 x 1 u - 6
= 24 u - 6

Number of chairs in the end
= 15 x (1 u + 11)
= 15 u + 165

The total number of chairs is equal.

24 u - 6 = 15 u + 165
24 u - 15 u = 165 + 6 
9 u = 171

1 u = 171 ÷ 9 = 19 

Number of chairs in the end
= 15 u + 165
= 15 x 19 + 165
= 450

Answer(s): 450