The reading of an ideal voltmeter in the circuit shown is,

Bridge is balanced since \( \frac{20}{30} = \frac{20}{30} \) or \( \frac{R_{AC}}{R_{CB}} = \frac{R_{AD}}{R_{DB}} \) i.e., potentials at points C and D are equal. Thus, the reading of the voltmeter connected between C and D is 0 V.