得用if
#include <stdio.h>
int main()
{
int n,s,ts=0;
double q = 0;
scanf("%d",&n);
while(n!=-1)
{
scanf("%d",&s);
ts += s;
if(n>=90)
q += 4 * s;
else if(n>=85)
q += 3.7 * s;
else if(n>=82)
q += 3.3 * s;
else if(n>=78)
q += 3.0 * s;
else if(n>=75)
q += 2.7 * s;
else if(n>=73)
q += 2.4 * s;
else if(n>=68)
q += 2.0 * s;
else if(n>=64)
q += 1.5 * s;
else if(n>=60)
q += s;
scanf("%d",&n);
}
printf("%.2f",q/ts);
}
自己上网搜一下呗