Question

Please help me out with this

Answer 1

first series the lead resistance and room(bulb) resistance and gat current through Circuit.
by this current get voltage across Bulb (or room).
for second case as the heater is on.
in the room Heater and Bulb are in parallel combination ( remember home appliances connection). So get the room's net resistance and then series with lead resistance.
Now see in parallel combination the voltage across each appliances remain same.
hence here the voltage across room will be the voltage across Bulb and so Heater. So get the voltage across room by using ohms law.
Take difference.This will be change in voltage across Bulb.