Inserting elements to array in C

My task is: If we look at any two neighbour values in an array, if the one on the right is two times greater than the one on the left, their average should be inserted between them and the new array consisting of old and new elements should be printed. I have a problem with moving the other elements after average.And using special functions or libraries is not allowed.I am beginner, and I hope you could help.

#include <stdio.h>

int main() {
    int n, i, j;
    double a[100], average;

    printf("Enter the number of elements: ");
    scanf("%d", &n);
    for (i = 0; i < n; i++) {
        scanf("%lf", &a[i]);
    }
    for (i = 0; i < n; i++) {
        if ((a[i + 1] / a[i]) == 2) {
            for (i = j = 0; i < n; ++i)
                b[j++] = a[i];
            if (a[i + 1] / a[i] == 2) 
                average = (a[i + 1] + a[i]) / 2;
            b[j++] =average;
        }
    }
    for (i = 0; i < j; ++i) {
        printf("%lf\n", b[i]);
    }
}

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