Pipe A can fill a

Pipe A can fill the cistern in : 6 hours

Pipe B can fill the cistern in : 8 hours

Part of cistern filled by pipe A in one hour:

Part of cistern filled by pipe B in one hour:

Part of cistern filled by pipe A and pipe B together in one hour: =

They can fill the cistern together in hours.

Pipes A and B filled cistern together for 2 hours,

So part of the cistern filled pipes by A and B in 2 hours : =

Part of cistern which is empty =

Number of hours taken by pipe B to fill left over part of cistern : = ( here 8 is hours required by pipe B to fill the remaining part of the cistern).

Time taken by pipe B to fill the remaining part of the cistern: = = 3 hours 20 min