A tank holds 500 L of water with 20 kg of salt dissolved in it. Pure water flows in at 10 L/min and the well-stirred mixture flows out at the same rate. If y kg is the salt in the tank after t minutes, then dy/dt equals
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Hint
Rate out = (salt per litre) × (litres leaving per minute).
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Answer: C. −y/50
No salt comes in. Salt leaves at (y/500) × 10 = y/50 kg per minute, so dy/dt = 0 − y/50 = −y/50.