A cell of emf 90V is connected across series combination of two...

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

The equivalent resistance of the above circuit is ${R}_{eq} = \left(\dfrac{400 \times 100}{400 + 100}\right) + 100 = 80 + 100 = 180Ω$

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

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