Question 963834
Father (f) is (=) 3 times the daughter's (3d) age.  f=3d must be one of the equations.  That eliminates A and C.<P>
10 years ago means 10 must be subtracted from things.  That eliminates B.  <P>
The correct answer is 10.  Father's age 10 years ago is f-10 and daughter's age 10 years ago is d-10.  He was 7 times her age so f-10=7(d-10).