First find the total number of monthly commutes: 5 days/week * 4 weeks/month = <<5*4=20>>20 days/month
Then multiply the distance of the commute one way by the two ways: 21 miles * 2 = <<21*2=42>>42 miles
Then multiply the number of commutes by the number of miles per commute: 42 miles/commute * 20 commutes = <<42*20=840>>840 miles
Then divide the number of miles by the car's gas mileage: 840 miles / 30 miles/gallon = <<840/30=28>>28 gallons
Then multiply that number by the cost of each gallon to find the total cost: 28 gallons * $2.50/gallon = $<<28*2.5=70>>70
Then divide the total cost by the number of friends to find the cost per person: $70 / 5 people = $<<70/5=14>>14/person
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