Challenge 5 Solution

Here's my solution:

Doubler.h

#ifndef DOUBLER_H
#define DOUBLER_H

void Doubler(int& n);

#endif  // DOUBLER_H

Doubler.cpp

#include "Doubler.h"

void Doubler(int& n) {
  n *= 2;
}

The tricky part here was making sure to use the & operator to pass n as a reference.