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Physics Question

Three identical cells, each of internal resistance R, are connected in series with an external resistor of resistance R. The current in the external resistor is I. If one of the cells is reversed in the circuit, what is the new current in the external resistor? (In terms of I)The answer is I/3 but i'm struggling to find how that is the case. it would be great if someone could help me with this.
forget about the internal resistances, the initial emf is 3V (emf 1 cell = V). When you reverse 1 cell, its emf reverse (= -V) so now the emf is V + V -V = V
Same resistor, V decrease 3 times, I decrease 3 times

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