Thank you for reporting, we will resolve it shortly
Q.
Ten identical cells each of potential $E$ and internal resistance $r$ are connected in series to form a closed circuit. An ideal voltmeter connected across three cells, will read
Given: Initial number of cells $\left(n_{1}\right)=10$ ;
Potential of each cell $=E ;$ Internal resistance of each cell $=r$ and final number of cells $\left(n_{2}\right)=3$
We know from the Ohm's law, total voltage of ten cells = $10 \times E=10 E$ and total resistance in ten cells $=10 \times r$ $=10 r $
Therefore current in the circuit $(I)=\frac{10 E}{10 r}=\frac{E}{r}$ or potential difference across three cells $=I \times 3 r=$ $\frac{E}{r} \times 3 r=3 E$
(Since the voltmeter is ideal, therefore it will read $3 E$ ).