Is this card number possible?

Python Advanced astronaut ages teen+ From CS Unplugged

The problem

Write a function is_valid_card(number). number is 16 digits as text. Check it with the Luhn method: from the RIGHT, double every SECOND digit (the 2nd, 4th ...), subtract 9 from any doubled value over 9, and add everything up. The number is valid if it has 16 digits and the total divides by 10. Give back True or False. (Made-up test numbers only.)

You will practice: check digits, Luhn method, validation

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CC BY-SA 4.0

Adapted from a CS Unplugged programming challenge by the University of Canterbury Computer Science Education Research Group, licensed CC BY-SA 4.0.

Original challenge · License: CC BY-SA 4.0

Changes: Rewritten in our own words as a function with tests, hints and steps, in Python and JavaScript, by Open Cosmos. This version is also CC BY-SA 4.0.