UPSC CAPF (AC) 2025 Paper I · Question 58

Question 58 of the UPSC CAPF (AC) 2025 Paper I (3 August 2025). Try it first, then open the answer and explanation.

1. CAPF 2025 · P-I · Q58 · General Mental Ability · Numbers & Series

   The unit digit of (3 × 1 + 4 × 2 × 1 + 5 × 3 × 2 × 1 + ... + 20 × 18 × 17 × ... × 2 × 1) is

   1. 2
   2. 3
   3. 4
   4. 5

   (3 × 1 + 4 × 2 × 1 + 5 × 3 × 2 × 1 + ... + 20 × 18 × 17 × ... × 2 × 1) का इकाई अंक है

   1. 2
   2. 3
   3. 4
   4. 5
   Show answer

   Answer: (4) 5

   उत्तर: (4) 5

   The terms are 3 × 1!, 4 × 2!, 5 × 3!, ..., 20 × 18!, and every factorial from 5! onwards ends in 0. So only the first four terms matter: 3 + 8 + 30 + 144 = 185, whose unit digit is 5.

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