The hydraulic braking system for the truck and trailer is set to produce equal braking forces for the two units. If the brakes are applied uniformly for 5 sec to bring the rig to a stop from a speed of 20 mph while moving down a 10 percent grade, determine the force P in the coupling between the trailer and the truck. The truck weighs 20,000 lbf and the trailer weighs 15,000 lbf.