An isolated capacitor of unknown capacitance has been charged to a potential difference of $100 \mathrm{~V}$. When the charged capacitor is disconnected from the battery and then connected in parallel to an uncharged $10.0-\mu \mathrm{F}$ capacitor, the voltage across the combination is measured to be $30.0 \mathrm{~V}$. Calculate the unknown capacitance.