text: Tap 1: 15 hours - 1 tank => 1 hour - (1/15) tank\nTap 2: 12 hours - 1 tank => 1 hour - (1/12) tank\nTap 1 and 2: 1 hour - (1/15) + (1/12) = (3/20) tank\n\nSince,\n(3/20) tank - 1 hour\n(3/5) tank - 1*(3/5)/(3/20) = (3/5)*(20/3) = 4 hours