A tank has three taps a, b and c. If a and b is open, it can fill the tank in 12 hours, If b and c is open it can fill the tank in 15 hours and if a and c is open it can fill the tank in 20 hours. If only tap a is open in how much time it will fill the entire empty tank alone.
1
20 hours
2
30 hours
3
25 hours
4
28 hours
5
32 hours