Credit card errors
This resource explains the check digit algorithm invented by Hans Luhn. This algorithm is widely used for both credit and debit cards to check small errors in the input of card numbers, using the final digit on the card as a check.
The instructions clearly show how to apply the algorithm, and invites students to try it out for themselves. The calculation involves doubling alternate digits, finding sums of up to 15 digits and finding the difference between that sum and the next multiple of ten.
The teachers' notes include ideas for extension activities and links to examples of other check digit algorithms.