How many kilowatt hours are used to light a 100 watt light bulb for 1 hour?

Remember that 1 kW = 1000 W

So 100 / 1000 = 0.1

100 W = 0.1 kW

You take the power - the wattage of the lamp, and multiply with how long it is lit. A 100 W bulb - 0.1 kW - will use 0.1 kWh in one hour. Or 1 kWh in 10 hours. Or 10 kWh in 100 hours.