Multiple choice

Tap A can fill a tank in 20 hours; where as Tap B can do so in 60 hours. Pure milk flows out of Tap A and water flows out of Tap B. For how much time should water be allowed to flow out of Tap B such that an 80% concentration of milk is obtained in the tank by the time it is completely full?

  1. 10 hours

  2. 12 hours

  3. 15 hours

  4. 20 hours

  5. 30 hours
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Let T be the time for Tap B. Tap A fills 1/20 per hour, Tap B fills 1/60 per hour. Total volume is 1. Milk volume is (1/20) * (T + x) = 0.8, where x is the time A runs alone. Since the tank is full, (1/20)*(T+x) + (1/60)*T = 1. Solving these yields T = 12.

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