A can do a piece of work in 25 days and B can finish it in 20 days. They work together for 5 days, then A leaves. In how many days will B finish the work??

Think of it this way:

Both of them are doing 5 days of work. They both do some fraction of it. 

5/25= A  1/5= 4/20

5/20= B 1/ 5/20

They do 9/20 of the work or 9 days of work. 

20-9=11

It will take B 11 days to do the rest of the work.

I hope this helps!
~kaikers