Taxi cab passengers are charged $2,.50 upon entering a cab. they are then charged $1.00 for every mile traveled. is it proportional?

No, because the charge in dollars is 2.50+m where m is the mileage. If it was proportional, then the charge would double if the mileage is doubled, but this is not the case. If m=5 miles then the charge is $7.50. If the charge were to double to $15 then m=12.5 miles, which is more than double the mileage.