In 10 years, A will be twice as old as B was 10 years ago. If A is now 9 years older than B, the present age of B is:

A. 19 years
B. 39 years
C. 29 years
D. 49 years
Answer: B

Explanation:  

Let B's age = x years.
Then, A's age = (x+ 9) years.
(x+9+10)=2(x - 10) hence x=39.
Present age of B = 39 years.