M and N are two taps can fill the tank in 12 hours and 15 hours, respectively. The taps are opened at the same time and if drain pipe is also opened, then it took 20 minutes more to fill the tank. When the tank is full, then in what time the drain pipes will empty the tank?
1
140 hours
2
240 hours
3
340 hours
4
420 hours
5
410 hours