An environmental study models the concentration of a pollutant, PPP, in a stream as P=ln(0.8t)P = \ln(0.8t)P=ln(0.8t), where t>0t > 0t>0 is the time in hours after a treatment process begins. Determine the exact value of the rate of change of the concentration with respect to time at the instant when t=2.5t = 2.5t=2.5.