From: https://www.allaboutcircuits.com/worksheets/millmans-theorem/

Calculate the voltage across the starter motor terminals of the “dead” car, and the current through the starter motor, while a second car is giving it a jump-start:

Regard the starter motor itself as a 0.15 Ω resistor, and disregard any resistance of the jumper cables connecting the two cars’ electrical systems together.

It's a little hard to make out the numbers, but I found another site with a clearer picture. The left battery is 10.2V, the right battery is 13.0V and the motor is 15V.

I don't understand the 15V on the starter motor. Is it even possible for it to have a greater voltage drop than either of the parallel batteries? And if it is, how do I use Millman's theorem on a component where I know the voltage, but not the resistance?

I tried just making both motors 0.15 ohms and ignoring the 15V, but I don't get the right answer.

 

I think the motor on the right is supposed to be the alternator that is continuously charging the right battery at 15v.

 

I think the motor on the right is supposed to be the alternator that is continuously charging the right battery at 15v.

Ohhhh, that makes sense! Thank you! The math checks out now.