Are you sure that is all there is to the problem? If the road goes directly to Grandma's house, and Grandma lives $\sqrt{36+ 16}= \sqrt{52}$ miles away, Ms Red could get there in $\frac{\sqrt{52}}{5}$ hours. That is the "least amount of time" she could take. But I suspect there is more to the problem, than that! Is there some geometry here? Where does the forest lie in relation to the two houses and the road?