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UPSC Prelims 2025 CSAT, question 20

Topic: Number system

Q20

What is the unit digit in the multiplication of 1×3×5×7×9×⋯×9991 \times 3 \times 5 \times 7 \times 9 \times \dots \times 999 ?

  1. (a) 1
  2. (b) 3
  3. (c) 5Correct answer
  4. (d) 9

Solution.

Answer: (c) 5

  1. The product 1×3×5×7×9×⋯×9991 \times 3 \times 5 \times 7 \times 9 \times \dots \times 999 is the product of all odd integers from 11 to 999999.
  2. Because the product includes the factor 55 and contains no even factors, the product is an odd multiple of 55, so its units digit is 55.