Lets say David is x.
Now Stan is 4 years less than 2x. (2x-4)
John is x+3.
Therefore:
x+2x-4+x+3=35
4x=36
x=9
2x-4=2*9-4=18-4=14
x+3=9+3=12
David is 9, Stan is 14, John is 12.
