Five years ago, my grandfather was five times as old as i was. three years from now, my grandfather will be three times as old as i will be. how old am i now?

Me (now) : x

5 years ago, me: x - 5
5 years ago, my grandfather: 5.(x - 5)
Now, my grandfather: 5.(x - 5) + 5 = 5x - 25 + 5 = 5x - 20 (I)

Three years from now, me: x + 3
Three years from now, my grandfather: 3.(x + 3) = 3x + 9
Now, my grandfather: 3x + 9 - 3 = 3x + 6 (II)

Now:
3x + 6 = 5x - 20
26 = 2x
13 = x
So, I am 13 years old.  :)