#include <iostream>
#include <stdio.h>
#include "simpio.h"
#include "strlib.h"
#include "random.h"
using namespace std;
 
#define LowerLimit 0
#define UpperLimit 200
#define StepSize 20
 
double CelsiusToFahrenheit(double c);
 
int main()
{
	int c;
 
	printf("Fahrenheit     Celsius");
	for (c = LowerLimit; c <= UpperLimit; c += StepSize)
	{
		printf("%3d       %f\n", c, CelsiusToFahrenheit(c));
	}
	system("pause");
}
 
double CelsiusToFahrenheit(double c);
{
	return (9.0 / 5.0 * c + 32);
}

...
double CelsiusToFahrenheit(double c);
...
int main()
{
...
}

double CelsiusToFahrenheit(double c) ; // <- remove the semicolon
{
	return (9.0 / 5.0 * c + 32);
}

#include <stdio.h>
to 
#include <cstdio>

system("PAUSE");

#include <iostream>
and
using namespace std;

#include <iostream>
#include <stdio.h>
#include "simpio.h"
#include "strlib.h"
 
#define GCD();
 
int main()
{
	int x, y, g;
 
	printf("1st number = ");
	x = GetInteger();
 
	printf("2nd number = ");
	y = GetInteger();
 
	printf("The GCD of %d and %d is %d", x, y, g);
}
 
int GCD(int x, int y)
{
	int g;
 
	g = x;
	while (x % g != 0 || y % g != 0)
	{
		g--;
		}
	return (g);
}

#define GCD();

int GCD(int x, int y);