Challenge 1: One Function Pointer, Pointed at Two Different Functions — Possible Solution
====================================================================

#include <stdio.h>

int add(int a, int b) {
    return a + b;
}

int subtract(int a, int b) {
    return a - b;
}

int main() {
    int (*operation)(int, int);

    operation = add;
    printf("add: %d\n", operation(10, 3));

    operation = subtract;
    printf("subtract: %d\n", operation(10, 3));

    return 0;
}

Output:

    add: 13
    subtract: 7

WHY THIS WORKS AS AN ANSWER
------------------------------
The single variable operation is declared once with the correct
function-pointer syntax (parentheses around *operation, matching the
chapter's own syntax warning), then reassigned between two different
functions with matching signatures -- calling operation(10, 3) after
each assignment dispatches to whichever function it currently points
at, demonstrating that the call syntax itself never changes even
though the actual behavior does.