Show one whole solution.

Here's a rule. Use it.

Is this number valid? Follow the rule exactly.

1. Starting from the rightmost digit and moving left, double every second digit;
2. if doubling gives a number greater than 9, subtract 9;
3. add up all the digits;
4. the number is valid if the total is a multiple of 10.

79927398713

Try it before you scroll. Most people get stuck on one of these:

• Does the last digit count?
• Every second from which end?
• Add 16, or 1 + 6?

The rule is complete and correct. It just isn't clear yet, because you've never seen it done.

Watch the clip

The clip shows the same rule, then one complete example, start to finish. Then it shows the rule again.

The name for it

One whole solution

Here's the same number, worked from start to finish. Read every step.

Step 1. Start at the right. Skip the last digit. Mark every second digit going left.

Step 2. Double the marked digits.

Step 3. If a doubled digit is over 9, subtract 9. Then add everything up.

Read the rule again

1. Starting from the rightmost digit and moving left, double every second digit;
2. if doubling gives a number greater than 9, subtract 9;
3. add up all the digits;
4. the number is valid if the total is a multiple of 10.

79927398713

It's the same rule, but now each line points to something you watched happen. "Every second digit" means the marked ones. "Greater than 9" means 16 and 18. The questions that stopped you before have answers.

This is the Luhn check, used on payment card numbers.

Now you try one

One digit changed: the last one. Is this number valid? Work it the same way, then check.

79927398710

Teach a procedure this way

1. Show one complete, correct example first, with every step visible.
2. Then state the rule, and point to where each part of it happened in the example.
3. Give a second problem with some steps done for them. Then one with none done.
4. Stop giving examples once people can do it. Experts learn more from solving problems than from reading worked ones (this is called the expertise reversal effect).

Ask your agent

Teach me the procedure below with the worked-example method.
1. Pick one realistic input and solve it completely, showing every step and every intermediate value.
2. Then state the general rule, and for each part of the rule, point to the step in the example where it happened.
3. Give me a second input with the first half done, and ask me to finish it. Show the answer only after.
4. Give me a third input to do on my own.

Procedure:
[paste here]

What good output looks like: a full example with every number shown, the rule mapped back onto it, then practice that fades the help.