عِشْرُونَ رَجُلًا / عِشْرِينَ رَجُلًا
Translation: Twenty men: the number changes its case, while the specifying noun «رَجُلًا» remains accusative.
عِشْرُونَ: nominative subject in «جَاءَ عِشْرُونَ رَجُلًا».
عِشْرِينَ: accusative object in «رَأَيْتُ عِشْرِينَ رَجُلًا».
عِشْرِينَ: genitive in «مَرَرْتُ بِعِشْرِينَ رَجُلًا».
Rule: With the tens 20–90, the counted specification is normally singular, indefinite, and accusative. These separate sentences are teaching adaptations of the source construction, not direct quotes.