自己写了一遍

#include<cstdio>
int main()
{
    double x,sum=0;
    scanf("%lf",&x);
    if(x<=2000)
    printf("Isento");
    else
    {
        if(x<=3000)
        sum+=(x-2000)*0.08;
        else if(x<=4500)
        sum+=1000*0.08+(x-3000)*0.18;
        else
        sum+=1000*0.08+1500*0.18+(x-4500)*0.28;
        printf("R$ %.2lf",sum);
    }
    return 0;
}

跟着Y总写了一遍

#include<cstdio>
int main()
{
    double x,sum=0;
    scanf("%lf",&x);
    if(x>2000)
    {
        double y=3000;
        if(x<=3000) y=x;
        sum+=(y-2000)*0.08;
    }
    if(x>3000)
    {
        double y=4500;
        if(x<=4500) y=x;
        sum+=(y-3000)*0.18;
    }
    if(x>4500)
    sum+=(x-4500)*0.28;
    if(sum==0) printf("Isento");
    else printf("R$ %.2lf",sum);
    return 0;
}