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?
+128460 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
