You cannot assume that the boys and girls who left equals 4 units.

Realise that this assumption is wrong as :

1 unit of girls left = 48. 3 units of boys left = 132
If 3 units of boys = 132, then 1 unit
= 132 ÷ 3 = 44 boys

Notice that 1 unit for both cases is different. It has to be the same, so this assumption is wrong.



One correct way to do this question : unit -part method

Boys at first : 5 units
Boys at the end : 2 parts
So 2 parts + 132 = 5 units

Girls at first : 2 units
Girls in the end : 1 part
So 1 part + 48 = 2 units

What we do is make the number of parts the same for both boys and girls and compare.

If 1 part + 48 = 2 units, multiply both sides of the equation by 2,

2 (1part + 48) = 2 x 2 units
2 parts + 96 = 4 units

Now recall from above that
2 parts + 132 = 5 units

Comparing these 2 equations, we realise that difference = 132 - 96 = 36
Difference also = 5 units - 4 units = 1 unit

So 1 unit = 36

Total number of boys and girls at first
= 2 units + 5 units
= 7 units
= 7 x 36
= 252