If it takes 15 hours to travel 900 miles then it gives you the fraction 15/900 where the numerator(top number) is 15 and denominator(bottom number) is 900.
Then to simplify it: since 15 is a common factor of itself(15) and 900, 15/15 = 1(1 hour) and 900/15 = 60(60 miles).
