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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
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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

 

 

cool cool cool

CPhill  Feb 7, 2018

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