PSLE
In a school hall, chairs were arranged in rows such that there were exactly 15 chairs in each row. For a concert, Riordan brought 9 more chairs into the school hall and rearranged all the chairs. There are now exactly 12 chairs in each row and 10 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 + 10
Value	15	12
Total value	15 u + 9	12 u + 120

Number of rows at first = 1 u
Number of rows in the end = 1 u + 10

Number of chairs at first after the 9 chairs were brought in
= 15 x 1 u + 9
= 15 u + 9

Number of chairs in the end
= 12 x (1 u + 10)
= 12 u + 120

The total number of chairs is equal.

15 u + 9 = 12 u + 120
15 u - 12 u = 120 - 9 
3 u = 111

1 u = 111 ÷ 3 = 37 

Number of chairs in the end
= 12 u + 120
= 12 x 37 + 120
= 564

Answer(s): 564