Question

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 chemical in the water.let f(t)=3(0.8)^t model the concentration of chlorine in parts per million cfter t days

C) about when will the concentration be 3.5ppm? What does that mean?