$3)$ a$)$ How much time does it take after S$1$ is closed

for the voltage across C$1$ to be $25$ V$?$

b$)$ How much time does it take after S$1$ is closed

for C$1$ to fully charge?

S$1$

Vs R$1$

R$=4\mathrm{.}7$ M $$ C$1$

C$=2\mathrm{.}2$ F$50$ V