//C++ function headers
#include <iostream> 
#include <cmath> 
using namespace std;  

//Prototypes of three functions
void inputFunction(double &poundsWeight, double &ouncesWeight);
void calculateWeight(double poundsWeight, double ouncesWeight, double &kilogramsWeight, double &gramsWeight);
void outputFunction(double poundsWeight, double ouncesWeight, double kilogramsWeight, double gramsWeight);

//Define constant values to variables
const double KILOGRAMS = 2.2046;
const double GRAMS = 1000;
const double OUNCES = 16;
//main method 
int main() 
{    

	double poundsWeight,ouncesWeight;

	double kilogramsWeight,gramsWeight; 
	char inputOption; 


	do 
	{ 
        //Call three functions
		inputFunction(poundsWeight, ouncesWeight);
        calculateWeight(poundsWeight, ouncesWeight, kilogramsWeight, gramsWeight);
        outputFunction(poundsWeight, ouncesWeight, kilogramsWeight, gramsWeight);
        //lets the user repeat this computation for new input values
        // until the user says he or she wants to end the program
		cout << "Repeat this computation for new input values?(Y/N): ";
		cin >> inputOption;
		cout << endl;
	}while(inputOption == 'Y' || inputOption == 'y');
	system("pause");
	return 0;
} 
//Read a valid input as weight in pounds and ounces
void inputFunction(double &poundsWeight, double &ouncesWeight)
{
	cout << "Enter pounds weight: "; 
	cin >> poundsWeight; 
	while(poundsWeight < 0)
	{
		cout << "Enter a valid number of pounds weight: ";
		cin >> poundsWeight; 
	} 
	cout << "Enter ounces weight: "; 
	cin >> ouncesWeight; 
	while(ouncesWeight < 0 || ouncesWeight >= OUNCES)
	{
		cout << "Enter a valid number of ounces weight: ";
		cin >> ouncesWeight; 
	} 
	cout << endl;
} 
//Calculate weight in kilograms and grams from the input values
void calculateWeight(double poundsWeight, double ouncesWeight, double &kilogramsWeight, double &gramsWeight)
{   
	
	double totalPoundsWeight;
	totalPoundsWeight = poundsWeight + ouncesWeight / OUNCES;
	kilogramsWeight =  totalPoundsWeight / KILOGRAMS;
	gramsWeight = kilogramsWeight * GRAMS;
	kilogramsWeight = floor(kilogramsWeight);
    gramsWeight = floor(gramsWeight - kilogramsWeight * GRAMS + 0.5);
} 
//Display output results on screen
void outputFunction(double poundsWeight, double ouncesWeight, double kilogramsWeight, double gramsWeight)
{
	cout <<"There are "<< poundsWeight 
    << " pounds and " << ouncesWeight 
    << " ounces are equal to " 
    << kilogramsWeight << " kilograms and " 
    << gramsWeight << " grams approximately." 
    << endl;

} 

Executed Output:

Enter pounds weight: 2000                                                                                                            
Enter ounces weight: 10

There are 2000 pounds and 10 ounces are equal to 907 kilograms and 478 grams approximately.                                          
Repeat this computation for new input values?(Y/N): Y

Enter pounds weight: 10000                                                                                                           
Enter ounces weight: 10
 
There are 10000 pounds and 10 ounces are equal to 4536 kilograms and 254 grams approximately.                                        
Repeat this computation for new input values?(Y/N): y 

Enter pounds weight: 100                                                                                                             
Enter ounces weight: 10 

There are 100 pounds and 10 ounces are equal to 45 kilograms and 643 grams approximately.                                            
Repeat this computation for new input values?(Y/N): N