So the first thing you want to do is convert Kelvin's total feet to miles: 3.75 miles
Now, see how many 28.2 seconds and 71.8 seconds are in a one hour.
60 times 60= 3600
3600/28.2=127.659574
3600/71.8=50.1392758
Now you want to multiply their times by the total miles they took.
127.659574 times 1.5= 191.5
50.1392758 times 3.75=188
191.5-188=3.5 miles
Tell me if this helps!!