Q. 
A is twice as efficient than B and together they can do the same work in as much time as C and D together. If C and D can complete the work in 20 and 30 days respectively working alone, then in how many days can A complete the work alone? 
A. 30 

B. 24 

C. 18 

D. 12 
A is twice as efficient than B and together they can do the same work in as much time as C and D together. If C and D can complete the work in 20 and 30 days
Let the total work = 60 units
C does 3 and D does 2 units per day
C & D together can complete in 60/5 = 12 days.
The efficiency ratio of A and B = 2 : 1
2 * x = (2 + 1) 12
x = 18 days