Zeph School had 520 pupils and Charlie School had 10% more pupils. When an equal number of pupils left each school, the ratio of the pupils in Charlie School to that in Zeph School became 5 : 4. How many pupils were there in both schools in the end?

	Zeph School	Charlie School	Difference
Before	520		52
Change	- ?	- ?
After	5 u	4 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 Zeph School and Charlie School at first and in the end the same. 

Difference between the number of pupils in both schools at first
= 10% x 520
= ¹⁰₁₀₀ x 520
= 52

Difference between the number of pupils in both schools in the end
= 5 u - 4 u
= 1 u

1 u = 52

Total number of pupils in both schools in the end
= 5 u + 4 u
= 9 u
= 9 x 52
= 468

Answer(s): 468