Alex and Mira starts from the same place walking at the rate of 5 miles/hr and 5.5 miles/hr respectively in the same direction. At what time will they be 8.5 miles apart ?

Question

$\begin{array}{1 1} (a)\;12 \;hrs \\(b)\;14\; hrs \\(c)\; 17 \;hrs \\(d)\;20\; hrs \end{array}$