In a mixing model, the salt in a tank after t minutes is y(t) = 300 + 60e^(−t/20) kg. How much salt is there in the long run?
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Hint
What happens to e^(−t/20) when t is very large?
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Answer: B. 300 kg
As t grows, e^(−t/20) gets closer and closer to 0, so y approaches 300 kg. The tank starts with 300 + 60 = 360 kg and never falls below 300 kg.