A tank contains 100 liters of salt solution with concentration 0.4 grams/liter. The tank is kept mixed as the same type of salt solution of concentration 1.4 grams/liter flows in at a rate of $r$ liters/min. Suppose the mixture is pumped out of the tank at rate $r$ liters/min, and $y(t)$ denotes the amount of salt in the tank in grams at time $t$ minutes.
If the concentration is 1.2 grams/liter after 60 minutes, find the value of the flow rate $r$ in liters/min.
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