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UPSC 2004: How many three-digit even numbers are there such that 9 comes as a succeeding digit in any number only when 7 is the preceding digit, and 7 is the preceding digit only when 9 is the succeeding digit?
(a)120
(b)210
(c)365
(d)405
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Not attemptedCorrect Answer: (d) 405
The rules force the pair “79” to occur only as an adjacent block (7 immediately followed by 9), and nowhere can 9 appear without a preceding 7 or 7 appear without a succeeding 9. Counting all admissible three-digit even numbers—those with no 7/9 at all plus those that begin with 79 and end in an even digit—gives 405 valid numbers, which matches option (d).([licchavilyceum.com](https://licchavilyceum.com/upsc-civil-services-prelims-2004-solved-question-paper/?utm_source=openai))
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Frequently Asked Questions
What is the correct answer to this UPSC 2004 Science & Technology question?▼
The correct answer is option (d): 405. Watch the detailed video explanation in our Sherlocking Prelims course on UnlockIAS.
Which year was this Quantitative Aptitude question asked in UPSC Prelims?▼
This question was asked in UPSC CSE Prelims 2004, GS Paper 1, as Question 136. It falls under Science & Technology > Quantitative Aptitude.
What is the difficulty level of this UPSC question?▼
This question is rated "difficult". It is a challenging question that demands deep subject knowledge and analytical thinking.
How many UPSC Prelims questions have been asked from Quantitative Aptitude?▼
UPSC has asked 178 questions from Quantitative Aptitude (under Science & Technology) across multiple years. Visit the topic page to see the full list and year-wise trends.