CDS 2026 (I) Elementary Mathematics, question 30

Topic: Number system

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Q30

What is the digit in the unit place of the number 11×33×55×77×991^{1}\times 3^{3}\times 5^{5}\times 7^{7}\times 9^{9} ?

  1. (a) 11
  2. (b) 33
  3. (c) 55Correct answer
  4. (d) 77

Solution.

Answer: (c) 55

  1. The factor 555^{5} ends in 5, and every other factor is odd.
  2. Multiplying a number ending in 5 by any odd number leaves the units digit 5.