If the required difference is 600 mi, lets start of counting by 10s and multiplying the current number 800-700 = 100 1600-1400 = 200 2400-2100 = 300 3200-2800 = 400 4000-3500 = 500 4800-4200 = 600 if 60 hours pass, they will be 600 miles away from each other. counting by 10 each time will help with problems like or similar to this.