Solution
= Solution
The <Hohmann transfer orbit> has <periapsis> $r$ and <apoapsis> $2r$, so its <semi-major axis> is $3r/2$. <Kepler's third law> gives a period $(3/2)^{3/2}$ times the original <orbital period>. Only half the <ellipse> is traversed:
$$
\boxed{t=\frac{T_0}{2}\left(\frac32\right)^{3/2}
\simeq22.0\,\mathrm h,\qquad T_0\simeq24\,\mathrm h.}
$$
The relevant radii are measured from <Earth>'s centre, not its surface.