Explanation:
By measuring the voltage drop before and after the resistors, and the current in the circuit, you can graph voltage vs current to get a line whose slope is equal to the equivalent resistance.
V = Req × I
For example, if you had two resistors in series, and each resistor was 1.5 Ω, then the equivalent resistance would be 3.0 Ω, and you would get the same graph as if there were a single 3.0 Ω resistor.