When two components are connected in series, where one is a fixed resistor and the resistance of the other can vary (e.g. thermistor, light dependent diode, variable resistor etc), you always make use of the fixed resistor to explain the its p.d. as resistance is constant.
Then use the concept of sum of p.d. across both components is equal to the e.m.f of the circuit to explain how the p.d. across the other component varies.
Though thermistor is not in SciPhy syllabus, it can still be tested as long as the information on thermistor is given. Refer to the videos below where 2 are from Sciphy and the other is from Pure.
Evan's Space 