Archie School had 640 pupils and Lee School had 20% more pupils. When an equal number of pupils left each school, the ratio of the pupils in Lee School to that in Archie School became 3 : 2. How many pupils were there in both schools in the end?

	Archie School	Lee School	Difference
Before	640		128
Change	- ?	- ?
After	3 u	2 u	1 u

Since an equal number of pupils left each school, the difference in the number of pupils at first and in the end between the 2 schools remains the same.  Make the difference between the number of pupils in Archie School and Lee School at first and in the end the same. 

Difference between the number of pupils in both schools at first
= 20% x 640
= ²⁰₁₀₀ x 640
= 128

Difference between the number of pupils in both schools in the end
= 3 u - 2 u
= 1 u

1 u = 128

Total number of pupils in both schools in the end
= 3 u + 2 u
= 5 u
= 5 x 128
= 640

Answer(s): 640