Engineering Board Exam Reviewer · Applications: Cooling, Heating, and Mixture Problems · medium

A 200-L tank contains 10 kg of salt dissolved in water. Pure water enters at 5 L/min and the well-mixed solution leaves at 5 L/min. How many minutes until only 2 kg of salt remain?

  1. 64.38 min
  2. 32 min
  3. 32.19 min
  4. 128.76 min

Answer

A. 64.38 min

Solution

dA/dt=-5A/200=-A/40 A=10e-t/40. Setting A=2: e-t/40= 15 t=40 5 64.38 minutes.

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