cash register program

hi pls gelp me with my assignment.. i supposed to create a cash register program that will calculate customer's payment and change..
here's what i have so far..

Code:

int main(void)
{

printf("Welcome to Reddish Bar&Cafe");
printf("\n\n\Choose from the items below:");

printf("\n\n\n\Margarita= 25 bucks");
printf("\n\Beer= 10 bucks");
printf("\n\Martini= 35 bucks");
printf("\n\Rhum= 20 bucks");
printf("\n\Tequilla= 25 bucks");
printf("\n\Shots= 10 bucks");

{
int a, answer, yes;
    
    printf("\n\n\ enter first item;");
    scanf("%d",&a);

    printf("do you want to add another another item?");
    answer=0;
    do{
        printf(" yes or no? ");
        scanf("%d",&answer);
        } 
        while(answer==yes);
        printf("\n\n\please enter another item");
}
printf("\n\n\n\Thank you and come back again!!");


return 0;
}

i cannot push thru.. i dont know to calculate as well. please help
thanks!!