Coding java program to calculate check digit numbers

I have this java project(using eclipse) that calculates check digit numbers. I'm stuck on how to code this one.

The check has a check digit so that it makes the sum of the sights including the check digit divisible by 7. Assume there are always 4 digits plus the check digit. The sample is 3875 with the number being 5 Second trial problem is 5862 and check number needs to be found. How do I go about doing this? I got to entering each digit and adding them but how can i do the rest?

This is for an into to computer science class so please no super complex stuff as if we didn't learn it I cant use it.

My teacher sucks by the way we learn none of this. I already did part a I need part b. Thanks. Here's an image to hell clarify.

First of all, you need to develop some "programmer logic", these problems help to develop it.

Airline tickets divide the actual identification number by 7 and assign the remainder to the check digit. Number can be of any length

Example:

12358 #3

Let's break this example:

12358 / 7 = 1765

and the reminder is 3

Let's do the same with the 2nd number on the example:

45349 / 7 = 45346

and the reminder is 3

So, your logic is correct.

An American Express traveler's check has a digit so that it maskes the sum of the digits, including the check digit, evenly divisible by 7.

Example:

3875 #5

In this problem the thing is a little different, you need to sum the digits:

3875 -> 3 + 8 + 7 + 5 = 23

Now you need to get the reminder of 23 / 7

23 / 7 = 3

And a reminder of 2

7 - 2 = 5

That's your checkDigit

5862

5862 -> 5 + 8 + 6 + 2 = 21

21 / 7 = 3

Reminder = 0

checkDigit = 7 - 0 = 7

So the formula is:

1. Split the number into digits
2. Sum the digits
3. Get the mod 7 of the sum
4. Make a rest of 7 - reminder