Suppose that the tractor-trailer truck of Prob. 7.100 is subjected to an unpowered, no-brakes no-deflector coast down

Question:

Suppose that the tractor-trailer truck of Prob. 7.100 is subjected to an unpowered, no-brakes no-deflector coast down on a sea-level road. The starting velocity is 65 mi/h. Solve, either analytically or on a computer, for the truck’s velocity V(t) and plot the results until the speed has dropped to 30 mi/h. What is the total coast down time?