PSLE
In a school hall, chairs were arranged in rows such that there were exactly 18 chairs in each row. For a concert, David removed 6 chairs from the school hall and rearranged all the chairs. There are now exactly 12 chairs in each row and 13 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 + 13
Value	18	12
Total value	18 u - 6	12 u + 156

Number of rows at first = 1 u
Number of rows in the end = 1 u + 13

Number of chairs at first after the 6 chairs were removed
= 18 x 1 u - 6
= 18 u - 6

Number of chairs in the end
= 12 x (1 u + 13)
= 12 u + 156

The total number of chairs is equal.

18 u - 6 = 12 u + 156
18 u - 12 u = 156 + 6 
6 u = 162

1 u = 162 ÷ 6 = 27 

Number of chairs in the end
= 12 u + 156
= 12 x 27 + 156
= 480

Answer(s): 480