What is the unit digit in the multiplication of 1 × 3 × 5 × 7 × 9 × ... × 999?

Quantitative Aptitude ·Previously asked in UPSC Civil Services Examination 2025

View the full solved paper: GS Paper II

Question

What is the unit digit in the multiplication of 1 × 3 × 5 × 7 × 9 × ... × 999?

  1. A. 1
  2. B. 3
  3. C. 5 (Correct answer)
  4. D. 9

Correct Answer

Option C — 5

Detailed Solution & Explanation

The correct answer is 5.

Key Points

  • The product 1×3×5×7×...×999 includes the factor 5 (and many multiples of 5 like 15, 25, 35...) as well as odd multiples.
  • Since 5 is a factor and no factor of 2 exists (all odd numbers), the product ends in 5. Answer: 5.