Gear A has 6 teeth and is rotating 20 RPM

Gear B has 12 teeth and is rotating ?

Gear C has 8 teeth and is rotating ?

What is the RPM’s of gear C?

First you find out the rpms of gear B

#teethB =           12 = 2 (Gear Ratio)
#teethA                6
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RPM A =             20= 10
GR                        2

Gear B is travelling 10 RPM

#teethC =            8 = .666
#teethB                12

RPM B =             10 = 15.01
GR                        .666

Gear C is traveling 15.04 RPM

Check your answers here

Gear Train : Gear Ratio, Torque and Speed Calculations