Question 253447
Since you didnt specify what the question was, I assume that you meant to find their ages. You could let mary be represented as x and joseph be represented as y. So you could set up two equations and substitute:

1)3y=x
2)x+y=29

You substitute equation 1 into 2. you get

3) 3y+y=29
simplify:
3)4y=29 
y:7.25

To find Mary's age, you could plug in your answer for Joseph into the first equation. 

1)3(7.25)= 21.75

So your answer would be:
Mary= 21.75 or 21 3/4 yrs old
Joseph= 7.25 or 7 1/4 yrs old