Determine the Thev$$nin equivalent circuit for the network external to the resistor R in Fig. $9\mathrm{.}140.$

Find the current through the resistor $R$ if its value is $20,50,$ and $100$

Without having the Th$$venin equivalent circuit, what would you have to do to find the current through the resistor R for all the values of part $(2)?$

Fig. $9\mathrm{.}140$