Discussion
An electricity utility company charges ₹ 7 per kWh (kilo watt-hour). If a 40-watt desk light is left on for 10 hours each night for 180 days, what would be the cost of energy consumption? If the desk light is on for 2 more hours each night for the 180 days, what would be the percentage-increase in the cost of energy consumption?
Answer: B
The cost is ₹ 504 and the increase is 20%: ₹ 504; 20%.
\[ \begin{aligned} E &= 40\ \text{W} \times 10\ \text{h} \times 180 = 72\ \text{kWh} \\ \text{cost} &= 72 \times 7 = 504 \end{aligned} \]
Two more hours a night makes 12 hours instead of 10. The cost is proportional to the hours, so it rises by 2/10 = 20% (to ₹ 604.8).
₹ 604.8 is the cost at 12 hours a night, not the original cost, which is the trap in two of the options. 12% and 15% do not follow from the ratio 12/10.
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