Prime Time — Quiz

Q1. If the idli-vada game is played from 1 to 60, in which set of numbers will the children say 'idli-vada'?

• A. {3, 6, 9, 12, 15, …}
• B. {5, 10, 15, 20, 25, …}
• C. {15, 30, 45, 60}
• D. {15, 30, 60}

Q2. At what number is 'idli-vada' said for the 10th time in the game?

• A. 75
• B. 100
• C. 150
• D. 300

Q3. Which of the following is true for every whole number greater than 1?

• A. It has at least three factors.
• B. 1 and the number itself are always factors of it.
• C. It is divisible by 2.
• D. It is prime.

Q4. Find the common factors of 20 and 28.

• A. 1, 2, 4
• B. 1, 4
• C. 2, 4
• D. 1, 2, 4, 5

Q5. In the treasure hunting game, Grumpy has kept treasures on 28 and 70. Which jump sizes will land on both numbers?

• A. Only 2 and 7
• B. 1, 2, 7 and 14
• C. 2, 7 and 14
• D. 1, 2 and 14

Q6. Which numbers in the list 23, 51, 37, 26 are prime?

• A. 23 and 51
• B. 37 and 26
• C. 23 and 37
• D. Only 37

Q7. How many prime numbers lie between 21 and 30 (inclusive)?

• A. 0
• B. 1
• C. 2
• D. 3

Q8. Ravi says 1 is a composite number because it has 'no other factors'. Is he correct?

• A. Yes, because 1 has only one factor
• B. Yes, because every whole number is composite
• C. No, 1 is neither prime nor composite
• D. No, 1 is the smallest prime

Q9. In the Sieve of Eratosthenes, what is done with the number 1 in Step 1?

• A. It is circled because it is prime
• B. It is crossed out because it is neither prime nor composite
• C. It is left alone
• D. It is circled because it is composite

Q10. Twin primes are pairs of primes

• A. That add up to a multiple of 5
• B. Having a difference of 2
• C. That look the same when reversed
• D. Whose product is a prime

Q11. Priya is asked to list twin primes between 1 and 100, other than (3, 5) and (17, 19). Which set is correct?

• A. (5, 7), (11, 13), (29, 31), (41, 43), (59, 61), (71, 73)
• B. (2, 3), (5, 7), (11, 13)
• C. (7, 11), (13, 17), (19, 23)
• D. (2, 4), (4, 6), (6, 8)

Q12. Are 4 and 15 a safe (co-prime) pair in the treasure-hunting game?

• A. No, both are composite
• B. No, they share the common factor 5
• C. Yes, their only common factor is 1
• D. Yes, because 15 is odd

Q13. Four different factor trees for 36. What is the unique prime factorisation in every case?

• A. 2 × 2 × 3 × 3
• B. 2 × 3 × 6
• C. 4 × 9
• D. 2 × 18

Q14. The rule for divisibility by 10. A number is divisible by 10 if and only if its last digit is

• A. 2 or 5
• B. 0 or 5
• C. 0
• D. Even

Q15. A number is divisible by 2 if its last digit is

• A. 0, 2, 4, 6 or 8
• B. 1, 3, 5, 7 or 9
• C. 0 or 5 only
• D. Any digit, since every number is divisible by 2