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  Choices:
A) 9 years
B) 19 years
C) 29 years
D) 39 years
E) none

Rationale: "solution let b ' s present age = x years . then , a ' s present age ‹ = › ( x + 9 ) years . ∴ ( x + 9 ) + 10 = 2 ( x - 10 ) ‹ = › x + 19 = 2 x - 20 ‹ = › x = 39 . answer d"
Answer: D