2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/128640Ulam asked for the maximum number of questions required to determine an integer between one and one million by asking questions whose answer is `Yes' or `No' and where one untruthful answer is allowed. Pelc showed that the number of questions required is 25. Here we give a simple proof of this result.Combinatorics94B99text