X can do a piece of work in 4 hours; Y and Z together can do it in 3 hours, while X and Z together can do it in 2 hours. How long will Y alone take to do it?  Choose the correct choice from the following answers.
A) 5 hours  B) 10 hours  C) 12 hours  D) 24 hours  E) 15 hours

Rationale: x 1 hour ' s work = 1 / 4 ; y + z ' s hour ' s work = 1 / 3 x + y + z ' s 1 hour ' s work = 1 / 4 + 1 / 3 = 7 / 12 y ' s 1 hour ' s work = ( 7 / 12 - 1 / 2 ) = 1 / 12 . y alone will take 12 hours to do the work . c
Answer: C