Thread: I need help with array please.

I'm new here so im not sure how this works and i apologise if i wrote anything wrong but the question is:
Read 25 integers entered via keyboard and store them in a file "num.dat". Next, read the file and store the values in an array;
double all the values in the odd positions and add 3 to the even positions in the array. Write the new values to another file, "endin.dat".

This is what i have an its obviously not working, can someone help me please.

Code:

#include<stdio.h>
int main (void){
    int num[25];
    int n;
    int sum1;
    int sum2;
    n=num[25];
    FILE * original = fopen("num.dat","w");
    printf("Enter 25 numbers:\n", num[25]);
    
    if (n%2==1){
        sum1=n*2;
        printf("Doubled odd numbers are %d\n",num[25]);
    }
    else {
        sum2=n+3;
        printf("Addition of three to even numbers %d\n", num[25]);
    }
    FILE * new = fopen("endin.dat","r");
    
    fclose(original);
    fclose(new);
    return 0;
}