Question

The ratio between the ages of father and son at present is 5:3 respectively.four years hence the ratio between the ages of the son and his mother will be 1:3 respectively.

Answer 1

10:3=x:15 
(solve for 'x')
so present age of dad is 50.
age difference between dad and son is 50-15=35
let 's consider,
 after 'n' years dad 's age is 'y'
y:y-35=2:1
solve for y, we get  y=70
dad's age after 'n' years- present age of dad=70-50=20
answer is 20