SOLUTION: Two cars leave an intersection at the same time. One heads east at an average speed of 40 mph and the other heads south at 30 mph. Find an expression for their distance apart d (in

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 theorem

So the expression for d is

Hope you got it :)