CBSE Class 6 · Computational Thinking · 2026-27

Prime Time: Worksheet

Skills: Algorithmic thinking, Pattern recognition, Decomposition. Circle the correct answer. Show your working in the space beside each question.

Which of these numbers is prime?
- (a) 51
- (b) 57
- (c) 59
- (d) 91
In 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?
- (a) 20
- (b) 25
- (c) 30
- (d) 45
Which is the prime factorisation of 84?
- (a) 2 × 42
- (b) 2 × 2 × 3 × 7
- (c) 4 × 21
- (d) 2 × 3 × 14
Which pair of numbers is co-prime?
- (a) 6 and 9
- (b) 8 and 15
- (c) 10 and 25
- (d) 14 and 21
A number is divisible by 4 if its last two digits are. Which number is divisible by 4?
- (a) 2,738
- (b) 3,516
- (c) 5,142
- (d) 7,350
Using 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?
- (a) 7
- (b) 11
- (c) 13
- (d) None — you are finished

Answer key: Prime Time

(c) 59 – 59 has no factors other than 1 and itself, so it is prime.
(c) 30 – The next one after 15 is 30.
(b) 2 × 2 × 3 × 7 – 84 = 2 × 2 × 3 × 7. The other options still contain 42, 4, 21 or 14, which are not prime.
(b) 8 and 15 – So 8 and 15 are co-prime.
(b) 3,516 – So 3,516 is divisible by 4.
(a) 7 – So you still need to cross out multiples of 7; then you are finished.