Let's assume that a month has four weeks.
If William made 400 miles by travelling only on Saturdays and Sundays and there were 4 weekends during the month, then he travels 400 / 4 = 100 miles during a weekend, which is 100 / 2 = 50 miles a day.
If Jason travels every weekday during the month and he does 500 miles, then he travels 500 / 20 = 25 miles a day.
It means that William travels more miles per day.
