= Solution
The <global warming potential> compares equal-mass pulses, while the tabulated <greenhouse gas radiative efficiencies> are per ppb. For a fixed emitted <mass>, the initial <mixing ratio> is inversely proportional to <molar mass>. The molecular formulas give approximately $M_r=151.39\ \mathrm{g\,mol^{-1}}$ for $\mathrm{C_2H_2Cl_3F}$ and $M_{11}=137.36\ \mathrm{g\,mol^{-1}}$ for $\mathrm{CCl_3F}$. Thus the ratio of mass-based <greenhouse gas radiative efficiencies> is $(0.24/0.26)(M_{11}/M_r)$.
In the standard one-box interpretation, the two quoted removal times represent independent first-order sinks acting on the burden. Their rates add, giving
$$
\tau_r^{-1}=\tau_{\rm trop}^{-1}+\tau_{\rm strat}^{-1},\qquad
\tau_r=\left(\frac1{0.75}+\frac1{20}\right)^{-1}=0.72289\ \mathrm{yr}.
$$
For exponential pulse removal, the time-integrated abundance is proportional to $\tau(1-e^{-H/\tau})$. Dividing the replacement's forcing integral by the CFC-11 integral cancels the unknown carbon-dioxide reference integral:
$$
\begin{aligned}
\mathrm{GWP}_{100,r}
&=5160\frac{0.24}{0.26}\frac{137.36}{151.39}
\frac{0.72289(1-e^{-100/0.72289})}{55(1-e^{-100/55})}\\
&\boxed{\simeq67.8\simeq68.}
\end{aligned}
$$
The finite-horizon factor for CFC-11 is important: $1-e^{-100/55}=0.8377$, not one. Omitting the molar-mass correction would incorrectly compare equal-ppb rather than equal-mass pulses.
This is about 76 times smaller than the stated CFC-11 value, because rapid tropospheric removal outweighs the fairly similar per-ppb <greenhouse gas radiative efficiencies>. It still represents appreciable warming per emitted <mass>. If the shorter lifetime alone is used as the troposphere-dominated approximation, the result is $\mathrm{GWP}_{100}\simeq70.4$, close to the combined-rate result. If the two given lifetimes instead describe separate local compartments, an exact two-box calculation would also require exchange rates and burden partitioning; those are not supplied. The boxed result states the usual one-box assumption explicitly.
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