Question 846766
let f = father's present age
let s = sum of the children's ages now
:
A father's age is three times the sum of the ages of his two children.
f = 3s 
In four years, he will be twice as old as the sum of their ages.
f + 4 = 2(s+8); both children will age 4 yrs
f = 4 = 2s + 16
f = 2s + 16 - 4
f = 2s + 12
Replace f with 3s from the 1st equation
3s = 2s + 12
3s - 2s = 12
s = 12 is the sum of his childen
then
f = 3(12)
f = 36 yrs is the father's present age