How does the energy used by a 100 W lightbulb compare with the energy used by a 40 W lightbulb in a given amount of time?

Power is the rate at which energy is used, so over the same amount of time, the energy used is proportional to the wattage. The ratio is 100 W ÷ 40 W = 2.5. Therefore, the 100 W bulb uses 2.5 times as much energy as the 40 W bulb in that time.

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