A can do a piece of work in 5 hours. B and C together can do it in 4 hours, while A and C together can do it in 3 hours. How long will B alone take to do it?  Pick one:
A) 8hr
B) 6hr
C) 9hr
D) 10hr
E) 12hr
A's 1 hour work = 1/5   B+9hr 1 hour work = 1/4   A+9hr 1 hour work = 1/3   A+B+9hr 1 hour work = 1/5 + 1/4 = 9/20   B's 1 hour work = 9/20 - 1/3 = 7/60 = 9hr (approximately)   B alone will take 9 hours approximately to do the work   Answer is 9hr
<result>C</result>
A can do a piece of work in 5 hours. B and C together can do it in 4 hours, while A and C together can do it in 3 hours. How long will B alone take to do it?