Question 215545
This problem ignores reality, however, lets plow thru it anyway

let d = dad's present age
Let s = sum of the children's present age
:
The age of a dad in years is three times the sum of his two children's ages.
d = 3s
:
 In four years time the dad is going to be two times older than the sum of his children's ages.
(d + 4) = 2(s + 4)
d + 4 = 2s + 8
d = 2s + 8 - 4
d = 2s + 4
:
 How old is the dad?
:
Substitute 3s for d in the above equation, find s
3s = 2s + 4
3s - 2s = 4
s = 4, sum of the children's ages
:
Then dad is 12!!