A and B can do a piece of work in 5 days, while C and D can do the same work in 20 days. In how many days will A. B, C and D do it together?

Correct option is B

Total Work = LCM of 5 and 20 = 20 unit

Efficiency of A and B = 20/5 = 4 unit per day 

Efficiency of C and D = 20/20 = 1 unit per day 

Total time taken by A, B , C and D = $$\frac{20}{4+1} = 4 \,days$$​​