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How many (4)-digit numbers can be formed from digits (1,2,3,4,5,6,7) without repetition such that (4) always appears to the left of (2)?अंकों (1,2,3,4,5,6,7) से (4) अंकों की ऐसी संख्याएँ कितनी बनेंगी जिनमें पुनरावृत्ति न हो और (4) हमेशा (2) के बाएँ आए?
Answer and explanationउत्तर और व्याख्या
Correct answer:सही उत्तर: (300)(300)
Choose (4) digits including (2) and (4), and in half of their arrangements (4) is left of (2). Symmetry saves time here.पहले (2) और (4) सहित (4) अंक चुनें और उनके आधे क्रमों में (4), (2) के बाएँ होगा। सममिति का उपयोग ऐसे प्रश्नों में समय बचाता है।
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