Question

We estimate that there are 256 million plug-in electric clocks in the United States, approximately one clock for each person. The clocks convert energy at the average rate of 2.2 W. To supply this energy, how many metric tons of coal are burned per hour in coal-fired electric-generating plants that are, on average, 22.4% efficient? The heat of combustion for coal is 33 MJ/kg. Answer in units of metric ton

Answer 1

256,000,000 * 2.2 W * 60s/min * 60min/hr = Energy needed in one hour.
That is simply the number of clocks times the power consumption times an hour. Remember that Watts is Joules / second so we needed to turn seconds into hours.
E= 2.02752*10^12

Now, that energy comes from coal at 33.0MJ/kg or 33,000,000. We need to multiply that by a number to factor in efficiency.
E= 2.02752*10^12 J/hr = 33MJ/kg * M * efficiency. Don't accidentally divide or put the efficiency on the wrong side. If you are in doubt, just make sure the efficiency makes the amount of coal needed increase, as it is less than 100% efficient.

2.02752*10^12 J/hr = 33MJ/kg * 0.224 * M
M = 274285.71kg = 274.28571 metric ton