Here is the program:
Code:
/* mpg.c computes the average miles per gallons for user entered number of trips */


#include<stdio.h>
 
int main()
{
    unsigned int gals;        // gallons of gas
    int miles;        // miles driven
    float mpg;        // miles per gallon per trip
    //int num_trips;    // total number of trips
    //float tot_mpg;    // avg mpg per number of trips
    float grand_avg_all_trips;         // average gas used for all trips


    // initialize
    int num_trips = 0;
    float tot_mpg = 0;


    // process and get data for first trip
    printf("%s", "Enter gallons used: -1 to end  ");
    scanf("%d", &gals);
    printf("%s", "Enter miles driven: ");
    scanf("%d", &miles);
    printf("The miles per gallon for this trip is %.2f\n", (float) miles/gals);
    


    while (gals != -1)
    {
        mpg = miles / gals;
        num_trips = num_trips + 1;
        tot_mpg = tot_mpg + mpg;


        // get data for next trip
        printf("%s", "Enter gallons used: -1 to end  ");
        scanf("%d", &gals);
        printf("%s", "Enter miles driven: ");
        scanf("%d",&miles);
        printf("The miles per gallon for this trip is %.2f\n", (float) miles/gals);
        
        
    }   // end while loop here


    // termination phase if at least one trip entered
    if (gals = -1)
    {
        grand_avg_all_trips = (float)tot_mpg / num_trips;
        printf("The average mpg for %d trips is %.2f: \n", num_trips, (float)tot_mpg / num_trips);
    }
    else
    {
        printf("No data was entered.");
    }
    return 0;


}

/*


Enter gallons used: -1 to end 8
Enter miles driven: 125
The miles per gallon for this trip is 15.62
Enter gallons used: -1 to end 5
Enter miles driven: 115
The miles per gallon for this trip is 23.00
Enter gallons used: -1 to end -1
Enter miles driven: -1
The miles per gallon for this trip is -0.00
Enter gallons used: -1 to end ^C










Enter gallons used: -1 to end 8
Enter miles driven: 125
The miles per gallon for this trip is 15.62
Enter gallons used: -1 to end 5
Enter miles driven: 115
The miles per gallon for this trip is 23.00
Enter gallons used: -1 to end -1
Enter miles driven:
0
The miles per gallon for this trip is 0.00
Enter gallons used: -1 to end -1
Enter miles driven: -1
The miles per gallon for this trip is -0.00
Enter gallons used: -1 to end




*/