First find the total payment James receives: $0.50/mile * 600 miles = $<<0.50*600=300>>300
Then find how many gallons of gas he needs to buy: 600 miles / 20 miles/gallon = <<600/20=30>>30 gallons
Then multiply that number by the price of gas per gallon to find the total cost of the gas: $4.00/gallon * 30 gallons = $<<4*30=120>>120
Then subtract that amount from James' earnings to find his profit: $300 - $120 = $<<300-120=180>>180
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