A ladder reaches a window which is 12 m above the ground on one side of the sheet,keeping its foot at the same point,the ladder is turned to the other side of the sheet to reach a window $9$ m high.The length of the ladder is $15$ m,then the width of the sheet is ______

Question

$\begin{array}{1 1}15\;m\\16\;m\\20\;m\\21\;m\end{array} $