[details=“English Translation”]After several minutes of explaining, the penny finally dropped for him.
[/details]
“caerle el veinte a alguien” appears to be an Americanism. I wonder if there’s a peninsular Spanish equivalent.
loc. verb. Mx, ES; Ho, pop + cult → espon. Darse cuenta tarde de algo o comprender sorpresivamente algo que era evidente.
(caer, caerse | Diccionario de americanismos | ASALE)
BTW, “ES” stands for El Salvador, not España.