Chlorine is frequently used to disinfect swimming pools. The chlorine concentration should remain between 1.5 and 2.5 parts per million (ppm) for safe swimming. After a warm, sunny day only 80% of the chlorine may remain in the water, with the other 20% dissipating into the air or combining with chemicals in the water. Let f(t)=3(0.8)^t model the concentration of chlorine in parts per million after t days. What is the initial concentration of chlorine in the pool?

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