Ann must a light bulb. She can choose between a 40W one for 0,60 euros or a 8W one for 4,20 euros. The elektricity is 0,20 euros per kWh. After how many hours is the second option cheaper to choose than the first option?

Guest Feb 7, 2018

#1**+1 **

40W = .04 kW

8W = .0008 kW

So....let the hours that equalize the cost = H......so we have

.60 + .04 (.20) H = 4.20 + .0008 (.20)H simplify

.60 + .008H = 4.20 + .00016H

Subtract .60 , .00016H from both sides

.00784H = 3.60

Divide both sides by .00784

H ≈ 460 hours

So....the second option is cheaper after ≈ 460 hours

CPhill Feb 7, 2018