Answer:
#include <iostream>
#include <iomanip> Â
using namespace std;
int main()
{
  double mpg, ppg;
  cout << "Enter your car's gas mileage (miles/gallon): ";
  cin >> mpg;
  cout << "Enter the gas price (dollars/gallon): ";
  cin >> ppg;
  cout << fixed << setprecision(2);
  for (int miles : { 20,75,500 }) {
    cout << miles << " miles will cost $" << miles * ppg / mpg << endl;
  }
}
Explanation:
I really like the new for(x:y) construct!