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    How to work it out? I've no idea.
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    The best thing to do is to just think of the voltmeters as 10MΩ resistors, as that's all they're doing in this situation. You can also treat R and V2 as a single resistor by using the formula for resistors in parallel Name:  resistance.png
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    If V1 has 4V, this means that the voltages are being shared in a 2:1 ratio between V1 and the R/V2 parallel bank. This means the ratio between their resistances must also be 2:1, so the total resistance of R and V2 must be half of the resistance of V1 (5V).

    This gets you  \frac{1}{5M\Omega} = \frac{1}{R} + \frac{1}{10M\Omega} which you can solve to get R=10MΩ
 
 
 
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