俺ならこう書く。

>>619
#include<stdio.h>
int main(void)
{
const int water=198;
const int milk=138;
const int money=1000;
const double tax=1.05;

int total = (int)((water + milk*2)*tax);
int answer = money - total;
printf("%d円¥n", answer);
return 0;
}

>>656
#include<stdio.h>
int main(void)
{
int price;
printf("定価＝");
scanf("%d",&price);
const int off[4] = {1, 3, 5, 8};
for(int i=0; i<4; i++){
printf("%d割引＝%d¥n", off[i], (int)(price*(1 - off*0.1)));
}

return 0;
}