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NDA 2 2024 Maths, question 3

Topic: Algebra

Q3

How many 4-digit numbers are there having all digits as odd?

  1. (a) 625625Correct answer
  2. (b) 400400
  3. (c) 196196
  4. (d) 120120

Solution.

Answer: (a) 625625

  1. Each position can independently contain 1, 3, 5, 7 or 9.
  2. None is zero, so the first position needs no restriction.
  3. Repetition is allowed because distinct digits are not required.
  4. The count is 5⁴=625.