20% work in 3 days So 100% work can be finished in 15 days.
80% of the work was completed by B in 20 days. So he can do the 100% work in 25 days
A+B B
15d 25d
LCM 75 So the efficiency of A and B is 5 Efficiency of B alone is 3. From that, efficiency of A is 2. Now the total work ÷ A's efficiency = 75÷2=37.5