= Solution
For a finite <group action>, the <orbit-stabilizer theorem> states
$$
|G|=|Gx|\,|G_x|,\qquad G_x=\{g\in G:gx=x\}.
$$
The bijection $gG_x\mapsto gx$ explains the formula by identifying cosets with the <group orbit>.
The <cube rotation group> acts transitively on its six faces. A rotation fixing one face fixes its center and its outward normal, so its axis is the line joining that center to the opposite face's center. The allowed rotations are the four multiples of $\pi/2$ about this axis. Thus the <stabilizer subgroup> is $C_4$, and
$$
\boxed{|G|=6\cdot4=24.}
$$
The <face-pair orbits of cube rotations> are the ordered equal-face pairs, opposite-face pairs, and adjacent-face pairs. These relations are preserved by every cube rotation. The first two families are transitive because the face action is transitive and the opposite face is uniquely determined. For the third, first send the first face to a prescribed face, then use its four quarter-turns to send the neighboring second face to any chosen neighbor. Thus there are exactly \b[three orbits], of sizes
$$
\boxed{6,\quad6,\quad24.}
$$
Their sizes sum to $36=|X\times X|$.
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