When i become as old as my father is now, i will be 5 times the age of my son. in addition, my son will be 8 yrs older than what i am now. if the sum of my father's age and my age is equal to100. how old is my son now?

Let f, s, m represent my father's age, my son's age, and my age, respectively.
.. f = 5s . . . . . . . . . . . . . . my father's age is currently 5 times my son's current age
.. s +(f -m) = 8 +m . . . . . when I'm as old as my father, my son will be 8 years older than I am now
.. f +m = 100 . . . . . . . . . the sum of my age and my father's is 100

These can be rewritten into a matrix form that makes solution using a calculator easier.
.. f -5s +0m = 0
.. f +s -2m = 8
.. f +0s +m = 100

A calculator shows the solution to be f=65, s=13, m=35.

My son is now 13 years old.