The estimated revenues r (in billions of dollars) from sales of digital music from 2002 to 2007 can be approximated by the model: r(t)=〖15.639t〗^3-104.75t^2+303.5t-3 01, t∈[2,7] where t represents the year, with t = 2 corresponding to 2002. Find the average of change of the model from 2002 to 2007. Interpret the result in the context of the problem.