Rohan knows "kitaab" (book) is feminine — "nayi kitaab" (new book)

Why

Rohan is reasonably hunting for a rule — things of the same kind should share a gender — but 'किताब' (book) and 'बस्ता' (bag) are both school items and still take different genders; no shared category decides gender in Hindi. Every noun's gender is fixed on its own, whatever group it belongs to. Test it: 'पेंसिल' (pencil, feminine) and 'रबड़' (eraser, masculine) are also both school items yet differ in gender — this is the rule, not an exception. No — 'बस्ता' is masculine, so 'नया बस्ता' is correct: sharing a category does not make words share a gender.