#include<stdio.h>
int main()
{
    int temp,a=1,b=2,num=1;
    double s,sum;
    sum=b/a;
    while(num<=20)
    {
        temp=b;
        b=a+b;
        a=temp;
        s=b/a;
        sum+=sum;
        num++;
    }
    printf("%.1lf",sum+s);
}

#include<stdio.h>
int main()
{
    int temp,a=1,b=2,num=1;
    double s,sum;
    sum=b/a;
    while(num<=20)
    {
        temp=b;
        b=a+b;
        a=temp;
        s=b/a;
        sum=sum+s;
        num++;
    }
    printf("%.1lf",sum);
}

答案是22的话就是上面这个代码
你的循环中sum+=sum有问题