$3.$

a$)$ Three capacitors of capacitances $\backslash (10\backslash$mathrm$\{$nF$\},20\backslash$mathrm$\{$nF$\}\backslash )$ and $30$ nF are connected to form a series network and an unknown voltage V is applied across the network. It is known that the charge stored on the $10$ nF capacitor is $60$ nC $.$ Determine the voltage across each capacitor. $(10$ points$)$

b$)$ The same three capacitors are connected in parallel and again some unknown voltage is applied across the network. It is known that the energy stored in the $10$ nF capacitor is $3\mathrm{.}0$ nJ $.$ Determine the electric charge stored in the other two capacitors. $(10$ points$)$