Derive an expression for the effective EMF and effective internal resistance of two cells connected in parallel.

In the given circuit, an ideal voltmeter connected across the $10 \mathrm{\Omega }$ resistance reads $2\mathrm{ }\mathrm{V}$ . The internal resistance $r$, of each cell is:

In the circuit shown in figure, reading of ammeter will: 

Eels can generate current with biological cells called electroplaques. The electroplaques in an eel are arranged in $100$ rows, each row stretching horizontally along the body of the fish containing $5000$ electroplaques. The arrangement is suggestively shown below. Each electroplaque has an emf of 0.15 V and internal resistance of $\text{0.25 Ω}$. What is the value of current through $\text{500 Ω}$ in given circuit?