My question is: The price of electricity went up by 25%. John's old computer used 800 watts and his new computer uses 50% more. If the old price of electricity was 12 cents per kilowatt-hour, how much does his computer cost, in dollars, to run for 50 hours? Your thoughts:
His new computer uses 800 * .5 = 400 watts more than his old computer. So it users 800 + 400 = 1200 watts. That is 1200 / 1000 = 1.2 kw. Electricity cost 12 * .25 = 3 cents more than it used to. So it now cost 12 + 3 = 15 cents per kwh. He used 1.2 * 50 = 60 kwh. So he paid 15 * 60 = 900 cents per kwh. So he paid 900 / 100 = $9. The final answer: 9.