How much does it cost to light your house? This is a very practical use of math. The equation to calculate the cost of lighting is:

C = (W/1,000) ´ R ´ N

Where

C = cost of lighting the bulb

W = label on the bulb in watts (W)

R = rate of electricity that shows on your electric bill $ / KWH

N = number of hours of usage

Note: 1/1,000 means to convert watts to kilowatts

Bill’s mother bought a 100-W light bulb for Bill’s room. He used the light about 4 hours per night. The electric rate in the city he lives in is $0.1 KWH. How much does his mother pay for the lighting bills for his room per month (30 days)?

C = (100/1,000) ´ ($0.1) ´ (4 ´ 30)

= $1.2

Bill’s mother must pay $1.20 for the light bill every month.

Bill's mother changed the lamp to a more efficient fluorescent light that shows 25-W on the label. By how much will the lighting bill change in a 30-day month?

Estimate the answer:

Less than 25¢
Between 25¢ and $1.00
Over $1.00