If he is 8 years older, then he will be twice as old when he is 16 and his sister is 8. Since that will be 3 years from now, he is now 13.
Algebraically....
let x be his current age
then x-8 is his sister's age
in 3 years his age will be x+3; his sister's age will be x-8+3 = x-5
then the problem says his age in 3 years will be twice his sister's age then:
