This is because if you draw a line going west and label it as 4 miles, then draw another line going south and label it 3 miles, you form a right angle, right? So, the fastest way to go back would be taking a diagonal-like route back to the beginning. Does this shape look familiar to you? Right! It's a right triangle. And do the numbers 3, 4, and 5 sound familiar to you? Yup, it's a Pythagorean triplet. If the legs of a right triangle are 3 and 4, the hypotenuse is 5. Why am I saying this? Because, in the drawing you just did, the 4 miles west and 3 miles south were the legs, and the diagonal path back to the beginning is the hypotenuse. Get it now?