My son is seven times older than my grandson and i am 12 times older than my grandson. if you add all our ages together, the sum is 100 years. can you tell me how old i am?

To solve this problem you first need to establish what it is you are looking for.. if you knew the grandson age then you could find your son and you.  So that is what we need to find and since we don't know it, we will call it x.

Grandson = x
Son = 7x
You = 12x

Then it says the sum of the 3 is 100 x + 7x + 12x = 100 then solve and find that x = 5.  So to determine your age, just put 5 in for x and you get 60... dang your old ha