Here -- would you put this on a high school math contest?

Mary is as old now as George will be when Mary finishes her time machine and goes to revisit George's birth. If future-Mary were to bring past-Mary along with her to the present day and meet now-Mary, the three of them would have an average age equal to George's present age. If Mary was six when George was born, how old is Mary now?

OK, maybe not.