Charlie School had 500 pupils and Will School had 30% less pupils. When an equal number of pupils joined each school, the ratio of the pupils in Will School to that in Charlie School became 4 : 3. How many pupils were there in both schools in the end?

	Charlie School	Will School	Difference
Before	500		150
Change	+ ?	+ ?
After	4 u	3 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 Charlie School and Will School at first and in the end the same. 

Difference between the number of pupils in both schools at first
= 30% x 500
= ³⁰₁₀₀ x 500
= 150

Difference between the number of pupils in both schools in the end
= 4 u - 3 u
= 1 u

1 u = 150

Total number of pupils in both schools in the end
= 4 u + 3 u
= 7 u
= 7 x 150
= 1050

Answer(s): 1050