Electricity costs ₹7 per kWh. A 40 W desk light runs 10 hours each night for 180 days. Find its energy cost, and the percentage increase if it runs two extra hours nightly for those days.
Energy=0.04 kW×10 h×180=72 kWh, costing ₹504. Running time rises from 10 to 12 hours, a 20% increase, and cost rises proportionally to ₹604.8. The requested original cost and increase are **₹504;20%, B**.
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