Ok, basically because you only have 2 equations, your choices for individual costs are various.
- Assume Oranges are free, then Apples would also be free and 1 bananna would cost £5.
- Assume Banannas are free, 1 orange costs £2.50, 1 Apple costs £2.50
- Assume Apples are free, Oranges cost nothing, Bannanas cost £5.
- Assume Bananas cost £4, Oranges cost 50p, Apples cost 50p
But 3 Oranges, 3 Banannas and 3 Apples would cost the same in all cases. They'd cost £15. It doesn't actually matter what each of them cost individually (it's actually impossible to work out) but knowing what they combine to cost can be done.