7/15 is greater than 9/20 due to the fact there being far more slices to the pie when you take 9 from 20 and there being less left when you take 7 from 15.
The greatest will always have less pieces remaining if not zero pieces remaining. If this was a question about fractions and how they are equal to one another it is a different story but I hope this was simple enough for you to draw a diagram for yourself.