This answer can be solved in many probablities,my prbablity is:-
let no of months of child be 90 so the no of years of the grandfather will be 90 years.
1 month has 30/31 days we will start counting the no of days from January then it would be 271 days and converting it to weeks will make it 38 weeks approximately.
let the age of son be x
let 140 represented by x
So,Age of grandfather=90 years
Age of father =38 years
Age of son =12 years