#include <stdio.h>
#include <string.h>
#define DENSITY 62.4
int main ()
{
    float weight,volume;
    int size,letters;
    char name[40];
    printf("Hi! what's your name?\n");
    scanf("%s",name);
    printf("%s, what's your weight in pounds?\n",name);
    scanf("%f",&weight);
    size = sizeof name;
    letters = strlen(name);
    volume =weight / DENSITY;
    printf("Well, %s, your volume is %2.2f cubic feet.\n ",name,volume);
    printf("Also,your first name has %d letters,\n",letters);
    printf("and we have %d bytes to store it in.\n",size);
    return 0;
}

具体些、是不能运行还是能运行没结果、看看文件建立的是否正确、

size = sizeof name;

怎么我运行的了……

size = sizeof name
不要括号的时候是在 sizeof int(数据类型的时候)

你数据类型名 要加括号

我也可以运行啊- -

@ 。@！！我也可以运行。。。

貌似没什么问题~~

修改完毕，如下
#include <stdio.h>
#include <string.h>
#define DENSITY 62.4
int main ()
{
    float weight,volume;
    int size,letters;
    char name[40];
    printf("Hi! what's your name?\n");
    scanf("%s",&name);//请加&
    printf("%s, what's your weight in pounds?\n",name);
    scanf("%f",&weight);
    size = sizeof(name);//请加()
    letters = strlen(name);
    volume =weight / DENSITY;
    printf("Well, %s, your volume is %2.2f cubic feet.\n ",name,volume);
    printf("Also,your first name has %d letters,\n",letters);
    printf("and we have %d bytes to store it in.\n",size);
    return 0;
}

估计是你建文件的时候步骤出错了，还有就是文件后缀名一定要为：（.CPP）