Sarthak Chhetri

Let the internal resistance of each cell be $R$ .

The $EMF$ across $AB=2V$ and Terminal voltage difference $(V)$ across $AB=1.6V$ .

The equivalent resistance $(R_{\text{eq}})$ of all the internal resistances $=\frac{R}{4}$ .

So current $(i)$ going through $15 || 15= \frac{15×15}{15+15}=\frac{15}{2}$ is $\dfrac{1.6}{\frac{15}{2}}=\frac{16}{75} A$ .

Now,

$\begin{aligned} EMF & =V+iR_{\text{eq}} \\ 2 & =1.6+\frac{16}{75}\cdot \frac{R}{4} \\ 0.4 & = \frac{4}{75} R \\ R & =\boxed{7.5} \end{aligned}$