Amovie star, unwilling to give his age, posed the following riddle to a gossip columnist: "6 years ago, i was 13 times as old as my daughter. now i am 7 times as old as she is." how old is the movie star?

To answer this question, you need to turn the description into an equation. If F is the father and D is the daughter, the equation would be:
1.  (F-6)= 13(D-6)
    F-6 = 13D -78
2. F= 7D
    D= 1/7F

If you put the 2nd equation into the 1st equation, you can determine the movie star(father) ages. The calculation would become: 
F-6= 13(1/7F)- 78
F-6 + 78=13/7 F 
13/7F - F= 72
6/7F= 72
F= 72 *(7/6)= 84 years