1 × 3 × 5 × 7 × 9 × ... × 999 के गुणन में इकाई अंक क्या है ?
Read this question in English
What is the unit digit in the multiplication of 1 × 3 × 5 × 7 × 9 × ... × 999?
(a) 1 · (b) 3 · (c) 5 · (d) 9
उत्तर और व्याख्या · explanation in English
Answer: (c) 5
The product of all odd numbers up to 999 is odd and contains 5 as a factor. An odd multiple of 5 always ends in 5.
- The product contains the factor 5, so it is a multiple of 5 and its unit digit is 0 or 5.
- Every factor is odd, so the product is odd and cannot end in 0.
- Hence the unit digit is 5.
- Check: 1 × 3 × 5 = 15 already ends in 5, and multiplying any number ending in 5 by an odd number keeps the last digit 5.
Remember · Odd × 5 ends in 5; an even number times 5 ends in 0. Spot a factor of 5 before multiplying anything.
Question and answer: UPSC's official GS Paper II (2025, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). ·