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A hardware shop sells 4 identical LED bulbs for ₹512. A hostel warden needs 15 such bulbs. What will they cost?

Class 6Ratio and ProportionRatio and Proportion
  • A₹1920
  • B₹2048
  • C₹1792
  • D₹7680

Step-by-Step Solution

Cost of one bulb = ₹512 ÷ 4 = ₹128. Cost of 15 bulbs = = ₹1920. The trap is ₹7680, which multiplies the whole ₹512 by 15 — you must find the price of ONE bulb first, then scale. ₹2048 and ₹1792 are the costs of 16 and 14 bulbs.
Answer: A) ₹1920

Key idea: Unitary method

If 5 pens cost ₹40, what do 8 pens cost? Step 1 — find the price of **one** pen: ₹40 ÷ 5 = ₹8. Step 2 — multiply by the new count: 8 × ₹8 = ₹64. The unitary method works for any 'X-many of something costs Y, what about Z-many?' problem. It's just two steps: find the unit, then scale.

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