Bill takes 20 days to empty a 50-liter barrel of beer
Bill's rate = output/time = (50 liters)/(20 days) = 5/2 liters per day

Together with Henry, they can empty it in 14 days
Combined rate = output/time = (50 liters)/(14 days) = 25/7 liters per day

Bill's rate + (Henry's rate) = Combined rate
So we can write: 5/2 + (Henry's rate) = 25/7
In other words: Henry's rate = 25/7 - 5/2 = 50/14 - 35/14 = 15/14 liters per day

How many days will it take Henry to finish a barrel of beer by himself?
Time = output/rate = 50/(15/14) = (50)(14/15) = 46 2/3 days.

So, in 47 days, Henry would finish a barrel of beer

Answer: D