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Sunday Times Teaser 3147 – Noteworthy

by Brian Gladman on January 14, 2023

by Victor Bryant

Published Sunday January 15 2023 (link)

Apparently in Costa Lotta a single-digit percentage of banknotes are forgeries and so I have designed a marker pen which tests whether notes are genuine. I thought it would be quite useful to the banks because, on average, for every N uses it only gives an incorrect result once (where N is some whole number).

Unfortunately my design has been abandoned by the banks because it turns out that on average for every N occasions on which the pen indicates a forgery, only one of the notes will in fact be forged!

What is N?

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