Question 802918
let his sister's age now be S
such that Darren's age now can be given as D = S + 8

thus 

in 3 years D + 3

he will be twice as old as she will be then (S + 3)*2

so
D + 3 = (S + 3)*2
(S + 8) + 3 = 2S + 6
S + 11 = 2S + 6
11 - 6 = S = 5;

D = 5 + 8 = 13