I understand that voltmeters are supposed to have high internal resistances so that they won't draw much current. However, they are being attached parallel to the resistor anyways ...
The voltmeter is connected parallel to a component of the circuit. This means the resistance (or impedance) of that part of the circuit is going to change, which in general changes...
while doing this question, I kept in mind following things: 1) Current is same in series(that's why current through 2M-ohm is same through Rb) (2) formula of equivalent resistance ...
3) in overall circuit, I used KVL and then I found Rb.. (4) after Rb, I know the voltage across Rb so using this I can find the internal resistance of voltameter across Rb...