Ten yrs ago, man will be 3x while his nephew will be x. five years from now, man will be 4x while the nephew would be x. again.
3x+15 (because 10 and 5 yrs) is equal to 4x.
then, it will become 15=4x-3x, or simply, 15=x.
the nephew is 15 years old.
(i think. dont trust me omg im sleepy rn, so sorry if its wrong :(((()