= Solution
Retain the source's idealized solar radial trajectory. Use $\mu_\odot\simeq1.32712\times10^{11}\,\mathrm{km^3\,s^{-2}}$, $r_i=1.5\times10^9\,\mathrm{km}$ and $v_i=22\,\mathrm{km\,s^{-1}}$. At $r=1.8\times10^{10}\,\mathrm{km}$,
$$
\boxed{v=\sqrt{22^2+2(1.32712\times10^{11})\left(\frac1{1.8\times10^{10}}-\frac1{1.5\times10^9}\right)}\simeq17.94\,\mathrm{km\,s^{-1}}.}
$$
The <speed> has decreased because the probe gains <gravitational potential energy> while moving away from the <Sun>. This is the supplied simplified model, rather than a reconstruction of <Voyager 1>'s actual three-dimensional trajectory or planetary encounters.
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