Determine the number of teeth in gear D in a reverted gear train in which gear A and B mesh, and compound gear B-C is concentric with A. Gear C meshes with gear D. The modules of gears A and C are 6 mm and 5 mm, respectively. The number of teeth in gears A, B, and C are 24, 36, and 40, respectively.

In the differential gear of a car, the pinion mounted on the propeller shaft has 20 teeth and the crown gear has 100 teeth. The propeller shaft rotates at 600 rpm. When the vehicle is taking a turn, one wheel rotates at 140 rpm. Determine the percentage change in speed of the other wheel.

A rack-pinion system has a pinion with 24 involute teeth, 144 mm pitch circle diameter, addendum of 6 mm for both pinion and rack, and 20° pressure angle. Calculate the contact ratio of the rack and pinion mechanism.