To find the amount of electricity used by the 100-watt bulbs, we need to first calculate the total wattage of the bulbs by multiplying the wattage of each bulb by the number of bulbs: 100 watts/bulb * 3 bulbs = 300 watts
Next, we need to convert the time the bulbs were used from hours to seconds. Since there are 3600 seconds in an hour, the bulbs were used for 5 hours * 3600 seconds/hour = 18000 seconds.
Now we can calculate the total amount of electricity used by the 100-watt bulbs by multiplying the wattage of the bulbs by the time they were used: 300 watts * 18000 seconds = 5.4 x 10^7 joules
To convert this to kilowatt-hours, we need to divide by the number of joules in a kilowatt-hour, which is 3.6 x 10^6. This gives us a total of 1.5 kWh.
At a cost of 9¢ per kWh, the cost of using the 100-watt bulbs for 5 hours per day is 1.5 kWh * $0.09/kWh = $0.135 per day.
To find the amount of money that would be saved by switching to 60-watt bulbs, we need to repeat the above calculation using the wattage of the 60-watt bulbs instead of the 100-watt bulbs. The total wattage of the 60-watt bulbs is 60 watts/bulb * 3 bulbs = 180 watts. This means that the total amount of electricity used by the 60-watt bulbs would be 180 watts * 18000 seconds = 3.24 x 10^7 joules, or 0.9 kWh.
At a cost of 9¢ per kWh, the cost of using the 60-watt bulbs for 5 hours per day is 0.9 kWh * $0.09/kWh = $0.081 per day.
Therefore, the amount of money that would be saved by switching to 60-watt bulbs is $0.135 per day - $0.081 per day = $0.054 per day.