(II) In Fig. $26-58$ (same as Fig. $26-17$ a), the total resistance is $15.0 \mathrm{k} \Omega,$ and the battery's emf is $24.0 \mathrm{~V}$. If the time constant is measured to be $24.0 \mu \mathrm{s}$, calculate ( $a$ ) the total capacitance of the circuit and $(b)$ the time it takes for the voltage across the resistor to reach $16.0 \mathrm{~V}$ after the switch is closed.