Let us suppose my grandson age is x years.
Given my son is 7 times older my grandson so my grand son age =7x.
My age is 12 times older my grandson age so my age = 12x
Sum of all the ages is given as 100 years
We can form an equation with this information
x+7x+12x=100
Adding all the like terms
20x=100
Dividing both sides by 20
x=5
The question is asking to find my age.
My age=12x=12(5)=60
My age is 60 years.