For every real number $x$, let $[x]$ be the greatest integer which is less than or equal to $x$. If the postal rate for first class mail is six cents for every ounce or portion thereof, then the cost in cents of first-class postage on a letter weighing $W$ ounces is always
$\text{(A) } 6W\quad \text{(B) } 6[W]\quad \text{(C) } 6([W]-1)\quad \text{(D) } 6([W]+1)\quad \text{(E) } -6[-W]$
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 $\fbox{E}$.