Solution: Let B finish this work X every day (that is, B's work efficiency is X), then

The single work efficiency is:1/4-x; C's personal work efficiency is: 1/5-x, so

2( 1/4-x+ 1/5-x)+6x = 1

Solution: x= 1/20

1/( 1/20)=20 (days)

So it takes B 20 days to finish the work alone.

I am glad to solve the above problems for you, and I hope it will help your study! ≤、≥ ∠