The detail solution to this question is like this.

Let's Say father's age in year is Y yr.Son's age in weeks is  x week,which if converted to year is 7x/365yr.Now as per the question,

        The Father replies, "My son is as many weeks as my grandson is in days

so grandson's age in days is X days.,which if converted to year is x/365 yr.

So 1st equation: y+7x/365+x/365 = 100....................(1)

Father says My grandson is as many months old as I am in years.

means,grandson is y months old,b'coz father is y yrs old.So equating grandson's age in days(x days) from above statement with his age in months (y month),both converted in years, gives us

x/365 = y/12.......................(2)

solving these two equations gives y=60 in years,that is father' age.grandson is as many months old as father is in years.So grandson's age is 60 months,i.e 5yrs,which converted to days is 1825 days.As son's age is as many weeks as grandson's age is in days.So son's age is 1825 weeks,that is converted in yrs is 1825*7/365=35yrs.

father     60yrs

son         35yrs

Grandson  5yrs