The calculation isn't hard. All you'd need would be some very fundamental arithmetic, a loop, and a branch. If you haven't done so, you should start by reviewing the basics thread stuck to the top of this forum.

You'll want to have done the following:

- get a basic hello world running

- exercise the arithmetic operators and print some results to the screen

- make a for loop that adds up the numbers from 1 to, say, 1000

- modify that loop to omit multiples of 13, that is, to only add number n if n is divisible without remainder by 13.

If you understand those things, you should be able to write this without a hitch, and all of those things you should be able to get at least a good start on from the tutorials.

This is what i came up with. I must be doing something wrong because its not working

```
import javax.swing.JOptionane;
//Author: Tony Fingers
//Purpose: Inputs the number of couples, singles who have RSVP'd and then calculates the total cost of decorations
public class Decorationcost
{
public static void main(String[]args)
{
int dom, number1, number2;
dom=0;
number1 = 0;
number2 = 0;
dom = Integer .parseInt(JOptionPane.showInputDialog("Enter the guests (1-100) who have RSVP'd."));
number1 = dom * 6;
number2 = dom * 10;
JOptionPane.ShowMessageDialog(null, "Single people who have RSVP'd is: "* number1 * "!");
JOptionPane.ShowMessageDialog(null, "Couples who have RSVP'd is: " * number2 * "!");
} //Closes class.
```

See the answer posted at http://www.java-forums.org/new-java/32528-birthday-party-code.html#post143795