Well, since you ran 4 miles, and Rob ran half as far, Rob ran 2 miles. Since Tim ran half as far as Rob, Tim ran 1 mile. Since Nicole ran half as far as Tim, Nicole ran 1/2 of a mile.
A. The expression is y=4/(2^x-1)
B. Nicole ran 1/2 of a mile.
Hope this helps!
-Raiden