Challenge 1: multiply, Prototyped Above main, Defined Below — Possible Solution
====================================================================

#include <stdio.h>

int multiply(int a, int b);  // prototype

int main() {
    int result = multiply(4, 6);
    printf("%d\n", result);
    return 0;
}

int multiply(int a, int b) {  // definition
    return a * b;
}

Output:

    24

WHY THIS WORKS AS AN ANSWER
------------------------------
The prototype above main gives the compiler multiply's full signature
before main ever calls it, satisfying C's top-to-bottom compilation
requirement even though the actual definition (with its real body)
appears later in the file, after main -- exactly the pattern the
chapter's own add() example demonstrates.