# include <stdio.h>
int main()
{int a=0,b=500;
while(a<=20)
{if(b%3==2&&b%5==2&&b%7==2)
printf("%d",b);
b++;
a++;
else
b++;
}
return 0;
}

if后面跟了三个语句，你是想闹哪样。。。用花括号括起来先

# include <stdio.h>
int main()
{
    int a=0,b=500;
    while(a<=50)
    {
        if(b%3==2&&b%5==2&&b%7==2)
            {
                printf("%d ",b);
                b++;
                a++;
            }
        else
             b++;
    }
    return 0;
}
亲，注意写代码的一些习惯哦

谢谢大家的赐教

唉，学点数论吧

#include<stdio.h>
int main()
{
    int i, k;
    k = (500 - 2 + 104) / 105;
    for(i = 0; i < 50; i++) printf("%d\n", (k + i) * 105 + 2);
    return 0;
}

else无法与if匹配呢，建议你用括号去体现你的逻辑