A take twice as much time as B or thrice as much time to finish a piece of work. Working together, they can finish the work in 4 days. B can do the work alone in?	Choose the correct choice.
A) 19	B) 12	C) 11	D) 24	E) 114

Rationale: "suppose a , b and c take x , x / 2 and x / 3 respectively to finish the work . then , ( 1 / x + 2 / x + 3 / x ) = 1 / 4 6 / x = 1 / 4 = > x = 24 so , b takes 12 hours to finish the work . answer : d"
Answer: D