The lamps in series are 10 volts 30 watts each, the equivalent resistances for these values are shown below. As you can see each lamp and resistance has a volt drop of 10 volts, added together it equals the supply voltage 5x10=50volts
What would happen if you substitute a lower wattage lamp in place of one of them, would it glow dimmer or brighter than the rest?
Try it, change the wattage of one to say 20 watts, then change the resistance below to equal the current flow through that lamp did the resistance become lower or higher?
