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Explanation

Because Gear B has more teeth than Gear A, one full rotation of Gear A will correspond to less than one full rotation of Gear B. This means Gear B must be rotating at a lower speed than Gear A.

The speed of gears in a gear train is inversely proportional to the number of teeth they have, based on the gear ratio. The gear ratio can be determined using the formula:
$$\mathrm{Gear}\mathrm{Ratio}=\frac{{\mathrm{Teeth}}_{\mathrm{Gear}\mathrm{A}}}{{\mathrm{Teeth}}_{\mathrm{Gear}\mathrm{B}}}=\frac{12}{36}=\frac{1}{3}$$
If Gear A rotates at 1800 RPM, Gear B will rotate at:
$$\frac{1800\mathrm{RPM}}{3}=600\mathrm{RPM}$$