since voltmeter resisitance is 400 ohms

R_ab = 400*400/800 = 200 ohms   , R_ad = 100  , R_cd= 100  ,R_bc =200 and R_ac =100 ohms

as R_ab/R_bc = R_ad/R_cd , by wheat stone bridge priciple, no current flows through Rac, so it can be removed from circuit

now, to fin volatge across, V , let I be current in circuit,

the on simplyfying resistance R_bd = 150 ohms

hence, I = 1/15 A

current in voltmeter = I/2 = 1/30 A

voltage across it = 1/30 * 200 = 20/3 V

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