1970 AHSME Problem 25

For every real number x, let [x] be the greatest integer 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

\textbf{(A) }6W\qquad \textbf{(B) }6[W]\qquad \textbf{(C) }6([W]-1)\qquad \textbf{(D) }6([W]+1)\qquad \textbf{(E) }-6[-W]

Full credit goes to MAA for authoring these problems. These problems were taken on the AOPS website.