A can work twice as fast as B. A and C together can work three times as fast as B. If A, B
and C complete a job in 30 days working together, in how many days can each of them complete the work.

The correct option is B.

Let the efficiencies of A ,B and C are a,b and c respectively .

     a :b = 2: 1   ....(1)

    (  a + c) : b = 3 : 1      ....(2)

 Comparing eq (1) and (2) we get a= 2 , b=1 and c = 1

 Total work = total efficiency x total time

                 = 4 x 30 = 120 units 

A alone does complete work in $$\frac{120}{2}$$ = 60 days.

 B alone does complete work in $$\frac{120}{1}$$ = 120 days.

 C alon does complete work in $$\frac{120}{1}$$ =  120 days.