楼主的程序做最小的改动，如下：

#include <stdio.h>
int main (void)
{
    float mile, gallon;
    printf ("Please enter your mile and gallon: ",mile,gallon);
    scanf ("%f %f", &mile, &gallon);
    const float L = 3.785 * gallon;
    const float KM = 1.609 * mile;
    printf ("Your car run %.1f mile per gallon.\n", mile / gallon);
    printf ("Which mean your car cost %.1f L per 100KM.\n", L *100 / KM);
    return 0;
}

然后结果应该是：
Please enter your mile and gallon: 500 10
Your car run 50.0 mile per gallon.
Which mean your car cost 4.7 L per 100KM.