Two automobiles start from a point A at the same time. One travels west at 72 miles per hour; the other travels north at 54 miles per hour. How fast is the distance between them increasing 3 hours after they start?

Is the product of two rational numbers irrational or rational? first, make a hypothesis by multiplying two rational numbers. then, use variables such as x=a/b and y=c/d and the closure property of integers to prove your hypothesis.