Need help with an Electronics Question, power and resistors!?

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Pinkie

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If electrical energy costs £0.15/kWh, for how many 23 hour days can a 5W nigh light be operated without the cost exceeding £1.00?

The answer is 55 days but i need the working out :(

x
 
5 W bulb working for 1 hour will need 5 Whr.
For 23 hrs, it will need 23 * 5 = 115 Whr
Since each KWhr costs .15 unit of money, for 1 unit of money u can use 1/.15 = 6.67 KWhr.

Per day the energy is .115 KWhr.
For 6.67 KWhr, it will take 6.67/.115 = 58 days


Ur ans is 55 days. You will get this answer if you light the bulb 24 hours a day (not for 23 hrs a day as in your question)
The right answer for your question is only 58 days.

Good luck!
 
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