The input shaft carries a 12-tooth external gear at 1,800 RPM. It meshes once with a 36-tooth driven gear on the output shaft. We want only the ideal tooth-count relationship; module, geometry, strength, efficiency, load, and maximum permitted speed must be checked elsewhere for the actual pair.
Divide driven by driver
The ratio is i = 36/12 = 3. Written as 3:1, this means the input makes three ideal rotations while the output makes one. The reduced exact fraction is the authoritative result. No decimal approximation is required in this example because 36 is exactly divisible by 12.
Use the reciprocal for output speed
The output speed factor is 1/i = 1/3. Multiplying 1,800 RPM by 1/3 gives exactly 600 RPM. The result assumes the input actually remains at 1,800 RPM and describes ideal kinematics only; it is not a promise about a motor under load or the safe speed of either gear.
One mesh gives opposite rotation
Because the two gears mesh externally once, the output rotates opposite to the input. The direction statement does not change the magnitude of the ratio. If an additional idler gear were inserted on its own shaft, the endpoint speed ratio could remain 3:1 while the extra mesh changed the output direction; that different topology is not represented by this one-stage example.