We have two responses for you

Alan,

Suppose you are an ant on Jane's shoulder. You are unaware that Jane is moving, you're just along for the ride. What you see is that Mary is moving away from you at 15 - 12 = 3 miles per hour. Three miles in an hour is one mile in a third of an hour or one half a mile in one sixth of an hour. One sixth of an hour is ten minutes and hence in ten minutes Mary is one half a mile in front of you. But this is on a half mile track so in ten minutes Mary is passing you.

I hope this helps,
Penny

Alan,

There is something wrong with the numbers. Nobody WALKS at 15mph, which is a rate of one mile every four minutes. I'm old enough to remember when a four-minute mile was something quite remarkable. No woman has ever RUN a 4-minute mile, let alone walked that fast. So let's put Mary and Jane on bicycles.

Chris