Practice questions

Reasoning practice questions (Group 4)

Questions 17 to 20 of 20

By TNPSC Guide editorial team · Last updated: 27.10.2026

1. Question 17

   Consider the following statements on cryptology. 1. The true message in its usual form is the plain text. 2. Cipher text is usually written in capital letters. 3. Plain text is usually written in capital letters. Which of these are correct?

   1. A. 1 and 2 only
   2. B. 1 only
   3. C. 2 and 3 only
   4. D. 1, 2 and 3
   Answer and explanation

   Correct answer: A. 1 and 2 only

   The message in its usual form is the plain text; cipher text is usually in capitals and plain text in small letters; so 1 and 2 are correct and 3 is wrong. (The textbook's own +4 table in Example 7.8 does the opposite, with the secret letters small and their meanings in capitals; statement 2 gives the usual rule.)

   Source: கணக்கு, எட்டாம் வகுப்பு, தமிழ் வழி (Mathematics, Standard VIII, Tamil medium; Samacheer Kalvi) (இயல் 7 தகவல் செயலாக்கம் (7.7 குறியாக்கவியல்), p. 262 (PDF p. 268))

2. Question 18

   A sequence is defined by a₁ = 2, a₂ = 2 and aₙ = aₙ₋₁ / (aₙ₋₂ + 2) for n ≥ 3. What is its 5th term?

   1. A. 1/8
   2. B. 1/20
   3. C. 1/11
   4. D. 1/16
   Answer and explanation

   Correct answer: B. 1/20

   a₃ = a₂/(a₁ + 2) = 2/4 = 1/2; a₄ = a₃/(a₂ + 2) = (1/2)/4 = 1/8; a₅ = a₄/(a₃ + 2) = (1/8) ÷ (5/2) = 1/20.

   Source: கணிதம், பத்தாம் வகுப்பு, தமிழ் வழி (Mathematics, Standard X, Tamil medium; Samacheer Kalvi) (இயல் 2 எண்களும் தொடர்வரிசைகளும் (2.6 தொடர்வரிசைகள்), p. 55 (PDF p. 61))

3. Question 19

   Consider the following statements on sequences. 1. A sequence with a finite number of terms is a finite sequence. 2. A sequence with an infinite number of terms is an infinite sequence. 3. Every function is a sequence. Which of these are correct?

   1. A. 1 only
   2. B. 2 and 3 only
   3. C. 1 and 3 only
   4. D. 1 and 2 only
   Answer and explanation

   Correct answer: D. 1 and 2 only

   Finitely many terms make a finite sequence and infinitely many an infinite one; every sequence is a function, but not every function is a sequence; so 1 and 2 are correct and 3 is wrong.

   Source: கணிதம், பத்தாம் வகுப்பு, தமிழ் வழி (Mathematics, Standard X, Tamil medium; Samacheer Kalvi) (இயல் 2 எண்களும் தொடர்வரிசைகளும் (2.6 தொடர்வரிசைகள்), p. 53 (PDF p. 59))

4. Question 20

   What is the next term of the sequence 1/3, 1/7, 1/11, 1/15, ...?

   1. A. 1/16
   2. B. 1/60
   3. C. 5/19
   4. D. 1/19
   Answer and explanation

   Correct answer: D. 1/19

   The numerator stays 1; the denominators 3, 7, 11, 15 grow by 4; the next denominator is 15 + 4 = 19, so the next term is 1/19.

   Source: கணிதம், பத்தாம் வகுப்பு, தமிழ் வழி (Mathematics, Standard X, Tamil medium; Samacheer Kalvi) (இயல் 2 எண்களும் தொடர்வரிசைகளும் (2.6 தொடர்வரிசைகள்), p. 54 (PDF p. 60))