Pipe A fills the cistern in half an hour and pipe B in 40 minutes, but owing to a crack in the bottom of the cistern it is found that pipe A now takes, 40 minutes to fill the cistern. How long will B take now to fill it and how long will the crack take to empty it ?

A)The leak empties in 1 hour and B fills in 2 hour
B)B fills in an hour and the leak empties in 2 hour
C)B fills in an hour and the leak empties in an hour

Can someone please tell how to solve this question and which rule is used in solving this??