Answer:

2 hours 24 minutes

Step-by-step explanation:

1st pipe fills in 4 mins

1st pipe fills 1/4 of the cistern in 1 min.

2nd pipe fills in 6 mins

2nd pipe fills 1/6 in 1 min

Together they will fill the cistern in 1 min=1/4 + 1/6= (3+2)/12 = 5/12 of cistern

They will fill the cistern in 2 hours 24 minutes.