A cell of emf 90   V is connected across series combination of two resistors each of 100   Ω…

As we know, voltmeter is connected in parallel to the element being measured.

The equivalent resistance of the above circuit is $R_{eq}=\left(\frac{400\times 100}{400+100}\right)+100=80+100=180 \mathrm{\Omega }$

Current through the circuit is $i=\frac{90}{180}=\frac{1}{2} \mathrm{A}$

Reading of the voltmeter is $V_R=i\left(\frac{400\times 100}{400+100}\right)=\frac{1}{2}\times 80=40 \mathrm{V}$