Hello.
To find the difference in cost per mile, divide the cost by the amount of miles, then subtract the two answers.
20.37 / 7 = 2.91 (cost per mile {cpm} for Uber)
15.40 / 4 = 3.85 (cost per mile {cpm} for Lyft)
3.85 - 2.91 = 0.94
The difference in cost per mile is $0.94