A commuter works 50 weeks out of the year, 5 days per week. That's 250 working days (commute days).

Not all days can be commuted due to weather conditions, family and personal appointments issues. Depending on what part of the country you live in. Not all workplaces are self contained; some workers need to use the car at work. So out of those 250 days there might be some days that simply won't work. Even at 10% loss, that's 225 days.