To find the unit price per ounce, you can divide the total cost by the number of ounces.
Rosie's hot chocolate costs $5 for a 20 oz cup.
So, the unit price per ounce is calculated as follows:
\[ \text{Unit price per ounce} = \frac{\text{Total cost}}{\text{Total ounces}} = \frac{5}{20} = 0.25 \]
Therefore, the unit price per ounce for her hot chocolate is $0.25 per ounce.