in the question it states the voltmeter is non ideal and has a resistance R, so u can basically assume it’s another resistor. therefore the circuit isn’t exactly series, and the total resistance would include the parallel combination of the voltmeter and 320 resistor + the 640 resistor. But since we don’t know the resistance of the voltmeter, we need to calculate the current in the 640 resistor as this will be the total circuit current.
hope that helped.