A mad doctor believes that baldness can be cured by warming the scalp with sound waves. His patients sit underneath the Bald-o-Matic loudspeakers, where their heads are bathed with 91dB of soothing 800Hz sound waves. Suppose we model a bald head as a 16-diameter hemisphere. If 0.23 of sound energy is considered an appropriate "dose," how many minutes should each therapy session last? Answer time in minutes