X and Y can do a piece of work in 15 days. Y and Z together can do it in 9 days. If Z is twice as good a workman as X, then find in how many days can Y alone does the work?

45

Z is twice as good a workman as X

i.e.     Z   :    X

         2    :    1

Let    4     :    2

as X + Y = 3R      →   Y = 3R - 2R = R

Z + Y = 5R       →      Y  = 5R - 4R = R

X   :   Y    :    Z

2   :   1    :    4

Total work = Efficiency × Number of days

Time taken by Y to do work   = \(\frac{45R}{R}\) = 45 days