Solution:
Consider two extreme cases. (i) When the resistance of the rheostat is zero, the current through O is zero since 0 is short-circuited. The circuit is then essentially a battery in series with lamp P. (ii) When the resistance of the rheostat is very large, almost no current flows through it. So, the currents through P and Q are almost equal. The circuit is essentially a battery in series with lamps P and Q.
