In the circuit shown, the switch S has been closed for a long time. At time $t= 0$, the switch is opened. It remains open for a long time T, after which it is closed again.

voltage drop across $100k\Omega$  resister is  $\frac{20e^{-t/1.5}}{3}V$ for t < T 

voltage drop across $100k\Omega$  resister is $10(e^{T-t}-e^{(T-3t)/3})$ for  $t>T$

voltage drop across  $100k\Omega$ resister is  $10e^{-t/1.5}V$ for $t<T$

The time constant of the circuit is 1.5sec for t > T