The question is easier than it appears at first.
Break down what you know...
We have only been told the cost of an adult and a child together, but nothing about the cost of a single adult or child.
There are therefore
The remaining amount of the entry fees is
This is entry fees paid by the
This means a child will pay
This could also have been solved by choosing variables to represent the cost of an adult and a child and forming two equations.
Solving these will lead to the same result