4595 hours
4mph
20/4 = 5 hours
Four miles per hour equates to 0.067 miles per minute.
540/4 = 132.5 hours
An average of 4mph.
Average walking speed is 4mph so around 17.5 hrs. but you may have to stop for food & drink,
It will take: 20/4 = 5 hours
About 15-16 hours (900 - 960 minutes) if you are walking at a normal pace. That's about 4mph. You would have to be fit to cover that distance in one go at that speed.
After three hours the first cyclist is 6 x 3 = 18 miles ahead. The speed of the second cyclict, when he starts, is 4mph relative to the first. So it will take him 18 miles/4mph = 4.5 hours to close the gap.
4mph
A brisk walk is about 4MPH, it would take 3hrs 18minutes at that pace.
6 miles @ 4mph would take 1.5 hours 6 miles @ 3mph would take 2 hours Total time = 3.5 hours