Identify given parameters

The porosity $n$ is given as 20%, which in decimal form is $n = 0.20$.

$n = 0.20$

State the relationship formula

The relationship between void ratio $e$ and porosity $n$ is given by the formula $e = \frac{n}{1-n}$.

$e = \frac{n}{1 - n}$

Substitute and solve

Substituting $n = 0.20$ into the formula, we get $e = \frac{0.20}{1 - 0.20} = \frac{0.20}{0.80}$.

$e = \frac{0.20}{0.80} = 0.25$