Two pipes P and Q can fill an empty tank in 25 hours and 20 hours respectively. Pipe R alone can empty the completely filled tank in 50 hours. Firstly both the pipes P and Q are opened and after 8 hours pipe R is also opened. What will be the total time (in hours) taken to completely fill the tank?
Two identical resistors, each of 10 ohm, are connected in parallel. This combination, in turn, is connected to a third resistor of 10 ohm. The equivalent resistance of the combination is ___.