Since it's 1.5 dollars per mile, you add 1.5 dollars for every mile and the amount of money total is 1.5m if m represents the amount of miles. In addition, since the flat tax is 15, you add 15 to it to get 1.5m+15. After that, since you only have 30 dollars, the amount of money is 30=15+1.5m. Subtracting 15 from both sides, we get 15=1.5m. Lastly, we divide both sides by 1.5 to get m=10