Question

An electronic gate is made of two doors each of which is five feet wide. When you push the gate open switch, both doors open inward on their hinges at the exact same speed. It takes 4 seconds for the gates to open completely so that the doors are perpendicular to the entryway. How fast is the distance between the ends of the doors changing two seconds after you push the switch.

(There's an answer that uses physics on Chegg, but I want a more calculus based answer)

Answer 1