A cell of EMF 4 V and internal resistance 0.5 Ω is connected to a 7.5 Ω external resistance. The terminal potential difference of the cell is

Ohm’s law applied to a total circuit can be written as $I=\frac{E}{R+r}$
where I is the current in the circuit, E is the EMF of the cell, R is the external resistance, r is the internal resistance of the cell. Terminal potential difference of the cell is given by V = E–ir.
Here, $E=4V,r=0.5\mathrm{\Omega },R=7.5\mathrm{\Omega }$
So, $\mathrm{I}=\frac{4}{7.5+0.5}=0.5\mathrm{A}$and terminal potential
differenceV = E - ir = 4 - 0.5 × 0.5 = 3.75 V.