Write out all but the last digit of the card number. Double every other number starting at the right. Sum the resulting digits (not numbers). E.g., if you doubled a 7 to become a 14 in Step 2, this ...
Katie has a PhD in maths, specializing in the intersection of dynamical systems and number theory. She reports on topics from maths and history to society and animals. Katie has a PhD in maths, ...
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