Yes… I don't get it.
It occurs in Pirx pilóta kalandjai (orig title. Opowieści o pilocie Pirxie; in Stanisław Lem teljes science-fiction univerzuma vol. 2, (2006) Szukits könyvkiadó, translator Murányi Beatrix), in the short story “Feltételes reflex”. The young student Pirx and the older scientist (astrophysicist) Langner are waiting in a hotel room on the moon, before traveling to the base at the back side of the moon.
Langner asks Pirx his age, without looking up from the complicated computations he was doing. Pirx replies “111”, which makes Langner look up, then adds “in binary”, which makes the scientist smile.
Langner kiterített az asztalon egypár új diagramot, és nagyítóval vizsgálgatta, olyan elmélyülten, ahogy Pirx még kedvenc színésznője képeit sem nézte soha. Közben megkérdezte Pirxtől, hány éves.
– Száztizenegy – felelte Pirx, és amikor társa felpillantott, hozzátette: – Kettes számrendszerben.
Langner most mosolyodott el először. Kezdett emberhez hasonlítani. Erős, fehér foga volt.
English translation (by Louis Iribarne) of that passage from Lem's novella "The Conditioned Reflex" as printed in the Avon Books edition of Tales of Pirx the Pilot:
Spreading out some new spectrograms on the table and studying them more intently than Pirx had had ever scrutinized his favorite pinup, Langner suddenly asked:
"How old are you?"
"A hundred and eleven," Pirx said, and added, when the other looked up, "in binary."
Langner broke out in a smile—his first—a smile that lent him an almost human look. He had strong, immaculately white teeth.
The numbers don't seem to make any sense. (Pirx is actually 22 years old at that time.) This appears to show that the young Pirx still says the first stupid thing that comes into his mind without thinking it through, which is in character for him. If there's a deeper meaning or joke behind the choice of these particular numbers, I can't see it.