Trick question---
Just kidding I think that 15 years ago his mother was 45
he was 15 and now he is 30 and she is 60.
literally sat here thinking that 30 is ruled out, and 60 would be 20 but then she'd be too old, suddenly it clicked, if you said he'd be half her age now, it'd be to easy, because half her age now would be 30 and it'd give the answer straight away.
You or your teacher has a big percentage of the brain used. I respect.