To represent the quantity of olives to pepperonis, we can use the numbers provided: Francis will use 20 olives from the can and 25 pepperonis from the package.
The ratio of olives to pepperonis can be written as:
\[ \text{Ratio} = \frac{\text{Number of olives}}{\text{Number of pepperonis}} = \frac{20}{25} \]
This ratio can be simplified by dividing both numbers by their greatest common divisor, which is 5:
\[ \frac{20 \div 5}{25 \div 5} = \frac{4}{5} \]
Therefore, the ratio of olives to pepperonis is 4:5.