Multiple choice

Water is flowing at the rate of 3 km/hr through a circular pipe of 20 cm internal diameter into a circular cistern of diameter 10 m and depth 2 m. In how much time will the cistern be filled?

  1. 1 hour

  2. 1 hour 40 min

  3. 1 hour 20 min

  4. 2 hour 40 min
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Cistern volume = π × (500 cm)² × 200 cm = 50,000,000π cm³. Pipe cross-section = π × (10 cm)² = 100π cm². Flow rate = 3 km/hr = 300,000 cm/hr, so volume per hour = 300,000 × 100π = 30,000,000π cm³/hr. Time = 50,000,000π ÷ 30,000,000π = 5/3 hours = 1 hour 40 minutes.

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