Averaging Errors

[Kid Programmer]

Hello guys. Whenever I am learning a new programming language I like to learn the basics (variables, loops, conditionals etc) by writing a program that will calculate averages. I wrote a simple number averaging program. But when I run it
in Eclipse or the command line and I enter:
1
3
5.5

It says the average is 3.1666666666666 665 when it should be 3.1666666666666 667. I checked and am positive it is dividing the sum by 3. What do I do? Here is my source:

/* Title: Number Averaging Program
 * Author: Kid Programmer
 * Language: Java
 * Integrated Development Enviroment: Eclipse
 * Description: This program will prompt a user to enter numbers.  When the user enters a double or string the program will calculate 
 * the average and print it out.  The program will then end.
 */
import java.util.Scanner;		//import a scanner

public class NumberAverager {	//create a class
	
	public static void main(String[] args) throws InterruptedException {		//start the main function
		double divideby = 0, number = 0, sum = 0, average = 0;			//declare variables
		Scanner scan = new Scanner(System.in);			//create a scanner named "scan"
		
		//explain the program
		System.out.println("  Loading finished.");
		System.out.println("This program will average numbers.");
		System.out.println("If you type with a number the program will not work.  If you type text before typing a number the program will not work.");
		System.out.println("To stop averaging numbers enter anything that is not a whole number.");
		System.out.println("Press enter after each number has been entered.");
		System.out.println("If the number is big, it will cancel the program.");
		System.out.println("Please enter the numbers: ");
		
		//create a loop to execute as long as the user enters numbers
		while ( scan.hasNextDouble() ) {
			number = scan.nextDouble();		//prompt the user for numbers
			sum = sum + number;				//add up the numbers
			divideby ++;					//increase the amount to divide the numbers by
			
			//if the user enters text
			if ( scan.hasNextDouble() == false) {		
				average = sum / divideby;			//calculate the average
				//print the average
				System.out.print("The total average of the numbers is ");
				System.out.print(average);
				System.out.println(".");
			}
		}
	}
}

[JosAH]

Read this: What every computer scientist should know about floating-point arithmetic.

kind regards,

Jos