Two charges $3 \times 10^{-8}$ C and $-2 \times 10^{-8}$ C are located 20 cm apart. At what point on the line joining the two charges is the electric potential zero?

The electric potential at a point is the algebraic sum of the potentials due to individual charges. For a point where the electric potential is zero, the contributions from positive and negative charges must cancel each other out. This occurs when the ratio of the charge magnitudes is equal to the ratio of their distances from the point: $\frac{|q_1|}{r_1} = \frac{|q_2|}{r_2}$. For two charges of opposite signs, there will be a point of zero potential between them (closer to the charge with smaller magnitude) and potentially another point outside the region between the charges (further from the charge with larger magnitude).