A takes 4 times as much time as B or 5 times as much time as C to finish a piece of work. Working together, they can finish the work in 4 days. B can do the work alone in:

10 days

efficiency ∝ \(\frac{1}{time}\)

                      A  :  B  :  C

 time               4  :  1

                      5         :  1 

                 ______________

                     20  :  5  :  4

efficiency         1  :  4  :  5  ⇒ 10

work = efficiency x time taken = 10 x 4 = 40 

time taken by B to complete the same work = \(\frac{40}{4}\) = 10 days