Their distance is the hypotenuse of a triangle with sides 5t and t which will be root((5t)^2^ + t^2^). So the distance at time t of the ex lovers will be root(26) × t. You can basically grasp intuitively that the speed is indeed constant and equals to the root(26)=5.1 ft/sec. Technically you’d use the derivative power rule to drop the t and get the speed.