• # question_answer If ${{N}_{a}}=\{an:n\in N\},$ then ${{N}_{3}}\cap {{N}_{4}}=$ A) ${{N}_{7}}$ B) ${{N}_{12}}$ C) ${{N}_{3}}$ D) ${{N}_{4}}$

${{N}_{3}}\cap {{N}_{4}}=\{3,\,6,\,9,\,12,15......\}\cap \{4,\,8,\,12,\,16,\,20,.....\}$    = {12, 24, 36......} = ${{N}_{12}}$. Trick: ${{N}_{3}}\cap {{N}_{4}}={{N}_{12}}$ [$\because$ 3, 4 are relatively prime numbers].