Answer:

1.   Her fastest time was between Checkpoints 3 and 4, when her speed was

2 miles per 5 minutes, or .4 miles per minute.

2.   Her slowest time was between Checkpoints 2 and 3 when the speed is 2 miles

per 15 minutes or .13 miles per minute.

or

1.      Her fastest time was between Checkpoints 3 and 4 when it took Sam

      5 minutes for 2 miles, or 2.5 minutes for each mile.

2.      Her slowest time was between Checkpoints 2 and 3 when it took her 15

     minutes to go 2 miles, or 7.5 minutes for each mile.