Let one pipe fills the cistern in x mins.
Therefore, the other pipe will fill the cistern in (x+3) mins.
Time taken by both, running together, to fill the cistern = 3 1/13 mins = 40/13 mins
Part filled by one pipe in 1 min = 1/x
Part filled by the other pipe in 1 min = 1/(x+3)
Part filled by both pipes, running together, in 1 min = 1/x + 1/(x+3)
Thus, one pipe will take 5 mins and other will take {(5+3)=8} mins to fill the cistern.