Mark is 11 years older than his sister. In 8 years he he will be twice as old as she will be then. How old are they now?
Mark is 14 years old and his sister is 3 years old.
Let's call Mark's age now
Let's call his sister's age now
We know now Mark is 11 years older than his sister or:
In 8 years, so
#m + 8 = 2(s + 8)
We already have the first equation in terms of
We can now substitute