1. The problem asks for an expression representing the total amount of drink Sandra has in millilitres, given she buys $p$ cans and $q$ bottles. 2. Each can contains 330 ml, so $p$ cans contain $330p$ ml. 3. Each bottle contains 500 ml, so $q$ bottles contain $500q$ ml. 4. The total amount of drink is the sum of the amounts from cans and bottles: $$\text{Total} = 330p + 500q$$ 5. This expression gives the total millilitres of drink Sandra has in terms of $p$ and $q$. Final answer: $$330p + 500q$$