smc

This question is trying to convert the floor function, which is more commonly notated as $\lfloor x\rfloor$, into the ceiling function, which is $\lceil x\rceil$. The identity is $\lceil x\rceil =-\lfloor -x\rfloor$, which can be verified graphically, or proven using the definition of floor and ceiling functions. However, for this problem, some test values will eliminate answers. If $W=2.5$ ounces, the cost will be $18$ cents. Plugging in $W=2.5$ into the five options gives answers of $15,12,6,18,18$. This leaves options $D$ and $E$ as viable. If $W=2$ ounces, the cost is $12$ cents. Option $D$ remains $18$ cents, while option $E$ gives $12$ cents, the correct answer. Thus, the answer is $\boxed{-6[-W]}$.