Since the cars are travelling east and south, which are perpendicular to each other, the paths of the 2 cars and the distance between them form the 3 sides of a right triangle, with the distance "d" being the hypotenuse. At the end of t hours, eastward car has travelled 40*t miles southward car has travelled 30*t miles Applying Pythagoras theoremSo the expression for d is Hope you got it :)