Question 815464
Let f = father's present age
let s = son's
:
Write an equation for each statement
:
A father’s age is four times that of his son.
f = 4s
:
After five years ,it will be three times that of his son.
f + 5 = 3(s+5)
f + 5 = 3s + 15
f = 3s + 15 - 5
f = 3s + 10
Replace f with 4s (from the the first statement)
4s = 3s + 10
4s - 3s = 10 
 s = 10 yrs is son's present age
then
4(10) = 40 yrs is father's age
:
 How many more years will take if father’s age is to be twice that of his son.
let y = no. of years, from the present, for this to be true
y + 40 = 2(y + 10)
y + 40 = 2y + 20
40 - 20 = 2y - y
y = 20 yrs from the now father will be twice as old as the son
:
Check this out
40+20 = 2(10+20)