The diagram shows a variable resistor R and two fixed resistors connected in series in a circuit to act as a potential divider. [Figure showing a potential divider circuit with a 6.0 V cell, variable resistor R, 2.0 Ω resistor, and 10 Ω resistor. A 2.0 V cell and galvanometer are connected across the 2.0 Ω and 10 Ω resistors.] The cell of electromotive force (e.m.f.) 6.0 V has negligible internal resistance. A cell of e.m.f. 2.0 V and a galvanometer are connected into the potential divider. The resistance of R is varied until the galvanometer reads zero. What is the resistance of resistor R?

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