In a series circuit, a 2 mA current flows. The supply is 10 V, and a voltmeter across R2 alone reads 6 V. What is the resistance of R2?

Correct answer: 3 kΩ

Use the voltage across R2 and the current through R2: 6 V / 2 mA = 3 kΩ. Using the 10 V total gives the whole circuit resistance, not R2.

Source: Lessons in Electric Circuits I: DC (Kuphaldt), 5.6: Correct use of Ohm’s Law