Question

Calculate the time constant (in second) of the circuit.

Answer 1

Answer: 0.12

$\tau =R_{Th}\cdot C;R_{Th}$ is the thevenin resistance at the capacitor terminals.
$R_{Th}=8+[20\Vert 9+(70\Vert 30)]=20k\Omega$
$\tau =0.12s$