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 7 days. B can do the work alone in?  Options:
A) 19 B) 12 C) 11 D) 42 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 / 7 6 / x = 1 / 7 = > x = 42 so , b takes 21 hours to finish the work . answer : d"
Answer: D