Because 30 + 30 equals 60 as he has to be half his age he'll be 30 but that's in 10 years time. you just subtract 30 and 60 by 10 and you'll get 20 and 50
Remembering the properties of numbers is important because you use them consistently in pre-calculus. The properties aren't often used by name in pre-calculus, but you're supposed to know when you need to utilize them.