Question 661005
If x = John's age:

Joke's age = 2x
Zainab's age = 2(2x) 


The sum is 336 so:

x + 2x + 2(2x) = 336
x + 2x + 4x = 336
7x*1/7 = 336*1/7
x*1 = 336/7
x = 48

If x=48, 
John is 48, Joke 96 and Zainab is 192.