The minimum number of identical bulbs of rating $4\text{ V}; 6\text{ W}$, that can work safely with desired brightness, when connected in series with a $240\text{ V}$ mains supply is :

The minimum number of identical bulbs of rating 4 V; 6 W that can work safely with desired brightness when connected in series with a 240 V mains supply can be calculated by dividing the total voltage by the voltage of each bulb. Therefore, the minimum number of bulbs is 240 V / 4 V = 60.