[ CBSE CLASS 6 / COMPUTATIONAL THINKING / CHAPTER 5 / 2026-27 ]

Class 6 Prime Time: Computational Thinking Questions and Answers

The Prime Time CT chapter covers factors, multiples, primes and divisibility through games and step-by-step methods like the Sieve of Eratosthenes. Students break numbers into prime factors (decomposition) and use divisibility rules as quick tests (algorithms).

Practise 6 questions Printable worksheet
Skills: Algorithmic thinking · Pattern recognition · Decomposition · Linked to Ganita Prakash Chapter 5: Prime Time

[ WORKED EXAMPLE ]

Which of these numbers is prime?

Answer: 59.

  1. 51 = 3 × 17 and 57 = 3 × 19.
  2. 91 = 7 × 13.
  3. 59 has no factors other than 1 and itself, so it is prime.

What this chapter is about

Students meet factors, multiples, common multiples, prime and composite numbers, co-primes and divisibility rules.

Games such as saying “idli” for multiples of 3 and “vada” for multiples of 5 make common multiples visible.

The Sieve of Eratosthenes is a classic algorithm: cross out multiples in order until only primes remain. Prime factorisation is decomposition in its purest form.

[ PRACTICE MODE / 6 QUESTIONS ]

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Pattern recognitionQuestion 1 of 6 · Score 0

Which of these numbers is prime?

Choose one answer

All Prime Time questions with answers

Try each one first, then open it to check the answer and the reasoning.

Q1Which of these numbers is prime?
  • 51
  • 57
  • 59
  • 91

Answer: C. 59

Hint: Try dividing each by 3 and by 7.

  1. 51 = 3 × 17 and 57 = 3 × 19.
  2. 91 = 7 × 13.
  3. 59 has no factors other than 1 and itself, so it is prime.

Skill: Pattern recognition · Level: medium · Report an error

Q2In the game, you say “idli” for multiples of 3, “vada” for multiples of 5, and “idli-vada” for multiples of both. After 15, what is the next “idli-vada” number?
  • 20
  • 25
  • 30
  • 45

Answer: C. 30

Hint: Multiples of both 3 and 5 are multiples of 15.

  1. A number divisible by both 3 and 5 is divisible by 15.
  2. Multiples of 15: 15, 30, 45 …
  3. The next one after 15 is 30.

Skill: Pattern recognition · Level: easy · Report an error

Q3Which is the prime factorisation of 84?
  • 2 × 42
  • 2 × 2 × 3 × 7
  • 4 × 21
  • 2 × 3 × 14

Answer: B. 2 × 2 × 3 × 7

Hint: Keep dividing by the smallest prime until every factor is prime.

  1. 84 ÷ 2 = 42, 42 ÷ 2 = 21.
  2. 21 ÷ 3 = 7, and 7 is prime.
  3. 84 = 2 × 2 × 3 × 7. The other options still contain 42, 4, 21 or 14, which are not prime.

Skill: Decomposition · Level: medium · Report an error

Q4Which pair of numbers is co-prime?
  • 6 and 9
  • 8 and 15
  • 10 and 25
  • 14 and 21

Answer: B. 8 and 15

Hint: Co-prime numbers have no common factor except 1.

  1. 6 and 9 share 3; 10 and 25 share 5; 14 and 21 share 7.
  2. 8 = 2 × 2 × 2 and 15 = 3 × 5 share no prime factor.
  3. So 8 and 15 are co-prime.

Skill: Abstraction · Level: medium · Report an error

Q5A number is divisible by 4 if its last two digits are. Which number is divisible by 4?
  • 2,738
  • 3,516
  • 5,142
  • 7,350

Answer: B. 3,516

Hint: Check only the last two digits of each number.

  1. 38, 42 and 50 are not divisible by 4.
  2. 16 = 4 × 4.
  3. So 3,516 is divisible by 4.

Skill: Algorithmic thinking · Level: easy · Report an error

Q6Using the Sieve of Eratosthenes up to 50, you have crossed out multiples of 2, 3 and 5. Which prime’s multiples must you still cross out?
  • 7
  • 11
  • 13
  • None — you are finished

Answer: A. 7

Hint: You only need primes whose square is 50 or less.

  1. 7 × 7 = 49, which is ≤ 50, so 49 must still be crossed out.
  2. 11 × 11 = 121 > 50, so multiples of 11 below 50 are already crossed out by smaller primes.
  3. So you still need to cross out multiples of 7; then you are finished.

Skill: Algorithmic thinking · Level: challenge · Report an error

Frequently asked questions

What does the Class 6 Prime Time Computational Thinking chapter cover?

The Prime Time CT chapter covers factors, multiples, primes and divisibility through games and step-by-step methods like the Sieve of Eratosthenes. Students break numbers into prime factors (decomposition) and use divisibility rules as quick tests (algorithms).

Which computational thinking skills does Prime Time use?

Prime Time mainly practises algorithmic thinking, pattern recognition, decomposition. It links to Ganita Prakash Chapter 5: Prime Time.

Where are the official CBSE Prime Time questions?

The official questions are in the CBSE Class 6 Computational Thinking and AI handbook for 2026-27, free on the CBSE Academic website. The practice on this page is original and mapped to the same chapter.

More Class 6 chapters