$($a$)$ Why can't we put two different magnitude current sources in series?

$($b$)$ Assume there are six light bulbs that can be connected in parallel and six different light bulbs that can be connected in series. In either case, each light bulb is to operate at $40$ W $.$ If the voltage at the plug is $115$ V for the parallel and series connections, analyze the circuit for the current through and the voltage across each bulb for both cases.

$($c$)$ The circuit in figure $6\mathrm{.}1$ is to control the speed of a motor such that the motor draws currents $5A,3A,$ and $1$ A when the switch is at high, medium, and low positions, respectively. The motor can be modelled as a load resistance of $20m.$ Solve it to find out the series dropping resistances $R_1,R_2,$ and $R_3.$