Question

how many hour must a 200 W bulb glows to consume 1 unit of electrical energy

Answer 1

since we know that 1 unit=1 kwh
also 1 kWh =1000wh
now we have a bulb that is of 200w
so we need to find the time to consume 1 unit of energy by it i.e 200=? hours 
to find the time taken we have to divide 100 wh by 200 w 
1000 wh/200w
hence the time taken to consume 1 unit of electrical energy is 5 hours.