arithmetic - two pipes a and b can fill a tank in 20 hrs and 30 hrs respectively if both the pipes are opened simultaneously how much time will be taken to fill the tank - skillgun

Part filled by A in 1 hr = 1/20
Part filled by B in 1 hr = 1/30
Part filled by (A + B) in 1 hr = 1/20 + 1/30
= (30 + 20)/600
= 1/12
Hence, both the pipes together will fill the tank in 12 hrs.