使用if选择结构 。学分=成绩*学分系数

float s,c,o;

if(s>=90){
    c=1.3; 
}else if(s>=80&&s<90){
    c=1.2;
}else if(s>=70&&s<80){
    c=1.1;
}else if(s>=60&&s<70){
    c=1.0;
}else{
    c=0.0;
}

o=s*c;

参考如下：

#include <stdio.h>
int main()
{
    int score,xf;
    float xs = 0;
    scanf("%d %d",&score,&xf); //从键盘输入成绩和学分，学分如果不需要输入可以直接写死 比如xf = 3;
    if(score>=90)
        xs = 1.3;
    else if(score >= 80 && score < 90)
        xs = 1.2;
    else if(score >= 70 && score < 80)
        xs = 1.1;
    else if(score >= 60 && score < 70)
        xs = 1;
    else
        xs = 0;
    printf("%.1f",xs*xf); //输出学分
    return 0;
}