David gets $$3$ per month as an allowance to spend any way he pleases. Because he likes only peanut butter and jelly sandwiches, he spends the entire amount on peanut butter $($at $$0\mathrm{.}05$ per ounce$)$ and jelly $($at $$0\mathrm{.}10$ per ounce$).$ Bread is provided free of charge by a concerned neighbor. David is a picky eater and makes his sandwiches with exactly $1$ ounce of jelly and $2$ ounces of peanut butter. He is set in his ways and will never change these proportions.