#include <stdio.h>
int main (void)
{
    const double unit_price = 3.50 ;
    int quantity = 0 ;
    printf("Enter the number that you want to buy :");
    sanf("%d",&quantity);
    if(quantity >10)
        printf("The price for %d is $%.2f\n",quantity,quantity*unit_price*0.95);
    else
        printf("The price for %d is $%.2f\n",quantity,quantity*unit_price);
    return 0;
}

Linking...
if1.obj : error LNK2001: unresolved external symbol _sanf
Debug/if.exe : fatal error LNK1120: 1 unresolved externals
执行 link.exe 时出错.

还有这种情况？

嗯，不晓得怎么回事！这个程序应该很简单，不应该出这个错误！

貌似打错了一个单词，休整就好了。