Gabriel School had 512 pupils and Charlie School had 25% less pupils. When an equal number of pupils joined each school, the ratio of the pupils in Charlie School to that in Gabriel School became 5 : 4. How many pupils were there in both schools in the end?

	Gabriel School	Charlie School	Difference
Before	512		128
Change	+ ?	+ ?
After	5 u	4 u	1 u

Since an equal number of pupils joined 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 Gabriel School and Charlie School at first and in the end the same. 

Difference between the number of pupils in both schools at first
= 25% x 512
= ²⁵₁₀₀ x 512
= 128

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

1 u = 128

Total number of pupils in both schools in the end
= 5 u + 4 u
= 9 u
= 9 x 128
= 1152

Answer(s): 1152