What is being suggested is not really a hard substitution here:
Code:
#include <iostream>
using namespace std;
int main()
{
 	int gallons, liters;
   

	cout << "Enter number of gallons: ";
	cin >> gallons;
    liters = gallons * 4;

	cout << "Liters: " << liters;
	
	system("PAUSE");
	return 0;
}
(Technically you need #include <cstdlib> at the top as well, I guess.)