#### Problem 79E

79. A balloon is rising at a constant speed of $5 \mathrm{ft} / \mathrm{s}$. A boy is cycling along a straight road at a speed of $15 \mathrm{ft} / \mathrm{s}$. When he passes under the balloon, it is $45 \mathrm{ft}$ above him. How fast is the distance between the boy and the balloon increasing 3 s later?