The present ages of a man and his wife are in the ratio 4 : 3. Four years from now the ratio of their ages will be 9 : 7. Find the man's present age.

Let the ages be $x$ and $y.x/y=4/3$ gives $3x-4y=0$. Four years later $(x+4)/(y+4)=9/7$ gives $7x-9y=8$. From the first, $x=4y/3$; substituting, $28y/3-9y=8$, so $y/3=8$ and $y=24$, giving $x=32$. Check: 32 : 24 = 4 : 3 and 36 : 28 = 9 : 7. 24 years is the wife's present age; 36 years and 28 years are the two ages four years from now, not now.