C 언어 책 문제 한번만 봐주세요!

#include <stdio.h>
#include <stdlib.h>
 
int main() {
  unsigned char *ptr = (unsigned char*) malloc(sizeof(unsigned char) * 30);
  ptr = ptr + 10;
  10[15 + ptr] = -10[ptr[-10]] + 5;
  return 0;
}

gcc x.c
x.c: In function ‘main’:
x.c:7:21: error: subscripted value is neither array nor pointer nor vector
    7 |   10[15 + ptr] = -10[ptr[-10]] + 5;
      |                     ^
 
Compilation exited abnormally with code 1 at Wed Jun 10 17:12:23

#include<stdio.h>
 
int main()
{
        int a[10];
 
        a[0]=100;
        a[1]=200;
 
        printf("%d %d %d %d\n",0[a],1[a],*(a+0),*(1+a));
 
        return 0;
}