GATE 2025 MN – Question 3

General Aptitude · Quantitative Aptitude: Data Interpretation, Arithmetic and Number Computation · 1 mark · Multiple choice

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?

₹ 604.8; 10%
₹ 504; 20%
₹ 604.8; 12%
₹ 720; 15%

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Correct answer: (B) ₹ 504; 20%

Explanation

Convert the lamp's power from watts to kilowatts: 40 W=0.04 kW. At ten hours per night for 180 nights, energy use is
$$E=0.04(10)(180)=72\ \text{kWh}.$$
At seven rupees per kWh, the cost is 72×7=**₹504**.

Two extra hours raise the daily duration from ten to twelve hours. With all other factors fixed, the cost rises by (12−10)/10×100=**20%**. The new cost is ₹604.80, confirming the increase. The requested pair is **B**.

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