Multiple choice

What will be the cost to plaster the inner surface of a well 14 m deep and 4 m in diameter at the rate of Rs. 25 per square meter?

  1. Rs. 4000

  2. Rs. 4200

  3. Rs. 4400

  4. Rs. 5400
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The inner surface of a well is the curved surface area of a cylinder: CSA = 2πrh, where r = 2 m (half of 4 m diameter) and h = 14 m. CSA = 2 × π × 2 × 14 = 56π ≈ 175.84 sq m. At Rs. 25 per sq m, total cost = 56π × 25 = 1400π ≈ Rs. 4400. Option A (4000) assumes π = 20/7, which is incorrect. Option B (4200) comes from using π = 3.

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