PSLE
In a school hall, chairs were arranged in rows such that there were exactly 20 chairs in each row. For a concert, Ethan removed 8 chairs from the school hall and rearranged all the chairs. There are now exactly 7 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	20	7
Total value	20 u - 8	7 u + 70

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

Number of chairs at first after the 8 chairs were removed
= 20 x 1 u - 8
= 20 u - 8

Number of chairs in the end
= 7 x (1 u + 10)
= 7 u + 70

The total number of chairs is equal.

20 u - 8 = 7 u + 70
20 u - 7 u = 70 + 8 
13 u = 78

1 u = 78 ÷ 13 = 6 

Number of chairs in the end
= 7 u + 70
= 7 x 6 + 70
= 112

Answer(s): 112