Question

the age of father is twice the sum of the age of his 2 children after 20 years his age will be equal to the sum of the ages of children find the age of father

Answer 1

let father's age be F and sum of children's age be S then
$f = 2 \times s \\ ie. \: s = \frac{f}{2}$
and
$f + 20 = s + 20 + 20 \\ ie. \: f = s + 20 \\ ie. \: f = \frac{f}{2} + 20 \\ ie. \: 2f = f + 40 \\ ie. \: f = 40$