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Solutions

Overview

Step-by-step NCERT solutions for Prime Time (Chapter 5, NCERT Class 6 Mathematics) — every question and answer worked out in full, not just the final result. You can also read the Prime Time textbook chapter.

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What these solutions cover

All 42 questions in Prime Time are solved in the PDF. Here's what's inside, exercise by exercise:

Common Multiples and Common Factors

  1. At what number is 'idli-vada' said for the 10th time? (In the Idli-Vada game, 'idli' is said for multiples of 3 and 'vada' for multiples of 5.)
  2. If the game is played for the numbers 1 to 90, find out: a. How many times would the children say 'idli' (including the times they say 'idli-vada')? b. How many times would the children say 'vada' (including the times they say 'idli-vada')? c. How many times would the children say 'idli-vada'?
  3. What if the game was played till 900? How would your answers change?
  4. Is the figure (the Venn diagram with circles for 'Multiples of 3' and 'Multiples of 5') somehow related to the 'idli-vada' game? Hint: Imagine playing the game till 30. Draw the figure if the game is played till 60.
  5. Find all multiples of 40 that lie between 310 and 410.
  6. Who am I? a. I am a number less than 40. One of my factors is 7. The sum of my digits is 8. b. I am a number less than 100. Two of my factors are 3 and 5. One of my digits is 1 more than the other.
  7. A number for which the sum of all its factors is equal to twice the number is called a perfect number. The number 28 is a perfect number. Its factors are 1, 2, 4, 7, 14 and 28. Their sum is 56 which is twice 28. Find a perfect number between 1 and 10.
  8. Find the common factors of: a. 20 and 28 b. 35 and 50 c. 4, 8 and 12 d. 5, 15 and 25
  9. Find any three numbers that are multiples of 25 but not multiples of 50.
  10. Anshu and his friends play the 'idli-vada' game with two numbers, which are both smaller than 10. The first time anybody says 'idli-vada' is after the number 50. What could the two numbers be which are assigned 'idli' and 'vada'?
  11. In the treasure hunting game, Grumpy has kept treasures on 28 and 70. What jump sizes will land on both the numbers?
  12. In the diagram, Guna has erased all the numbers except the common multiples. The common multiples shown are 24, 48, and 72. Find out what those numbers could be and fill in the missing numbers in the empty regions.
  13. Find the smallest number that is a multiple of all the numbers from 1 to 10, except for 7.
  14. Find the smallest number that is a multiple of all the numbers from 1 to 10.

Prime Numbers

  1. We see that 2 is a prime and also an even number. Is there any other even prime?
  2. Look at the list of primes till 100. What is the smallest difference between two successive primes? What is the largest difference?
  3. Are there an equal number of primes occurring in every row in the table (Sieve of Eratosthenes, numbers 1–100 in rows of 10)? Which decades have the least number of primes? Which have the most number of primes?
  4. Which of the following numbers are prime: 23, 51, 37, 26?
  5. Write three pairs of prime numbers less than 20 whose sum is a multiple of 5.
  6. The numbers 13 and 31 are prime numbers. Both these numbers have the same digits 1 and 3. Find such pairs of prime numbers up to 100.
  7. Find seven consecutive composite numbers between 1 and 100.
  8. Twin primes are pairs of primes having a difference of 2. For example, 3 and 5 are twin primes. So are 17 and 19. Find the other twin primes between 1 and 100.
  9. Identify whether each statement is true or false. Explain. a. There is no prime number whose units digit is 4. b. A product of primes can also be prime. c. Prime numbers do not have any factors. d. All even numbers are composite numbers. e. 2 is a prime and so is the next number, 3. For every other prime, the next number is composite.
  10. Which of the following numbers is the product of exactly three distinct prime numbers: 45, 60, 91, 105, 330?
  11. How many three-digit prime numbers can you make using each of 2, 4 and 5 once?
  12. Observe that 3 is a prime number, and 2 × 3 + 1 = 7 is also a prime. Are there other primes for which doubling and adding 1 gives another prime? Find at least five such examples.

Prime Factorisation

  1. Find the prime factorisations of the following numbers: 64, 104, 105, 243, 320, 141, 1728, 729, 1024, 1331, 1000.
  2. The prime factorisation of a number has one 2, two 3s, and one 11. What is the number?
  3. Find three prime numbers, all less than 30, whose product is 1955.
  4. Find the prime factorisation of these numbers without multiplying first: a. 56 × 25 b. 108 × 75 c. 1000 × 81
  5. What is the smallest number whose prime factorisation has: a. three different prime numbers? b. four different prime numbers?
  6. Are the following pairs of numbers co-prime? Guess first and then use prime factorisation to verify your answer. a. 30 and 45 b. 57 and 85 c. 121 and 1331 d. 343 and 216
  7. Is the first number divisible by the second? Use prime factorisation. a. 225 and 27 b. 96 and 24 c. 343 and 17 d. 999 and 99
  8. The first number has prime factorisation 2 × 3 × 7 and the second number has prime factorisation 3 × 7 × 11. Are they co-prime? Does one of them divide the other?
  9. Guna says, 'Any two prime numbers are co-prime'. Is he right?

Divisibility Tests

  1. 2024 is a leap year (as February has 29 days). Leap years occur in the years that are multiples of 4, except for those years that are evenly divisible by 100 but not 400. a. From the year you were born till now, which years were leap years? (Personal answer — varies by student) b. From the year 2024 till 2099, how many leap years are there?
  2. Find the largest and smallest 4-digit numbers that are divisible by 4 and are also palindromes.
  3. Explore and find out if each statement is always true, sometimes true or never true. You can give examples to support your reasoning. a. Sum of two even numbers gives a multiple of 4. b. Sum of two odd numbers gives a multiple of 4.
  4. Find the remainders obtained when each of the following numbers are divided by
    • (a) 10,
    • (b) 5,
    • (c) 2. Numbers: 78, 99, 173, 572, 980, 1111, 2345
  5. The teacher asked if 14560 is divisible by all of 2, 4, 5, 8 and 10. Guna checked for divisibility of 14560 by only two of these numbers and then declared that it was also divisible by all of them. What could those two numbers be?
  6. Which of the following numbers are divisible by all of 2, 4, 5, 8 and 10: 572, 2352, 5600, 6000, 77622160?
  7. Write two numbers whose product is 10000. The two numbers should not have 0 as the units digit.
Sample

Worked examples

3 questions from Prime Time, solved step by step.

Common Multiples and Common Factors

At what number is 'idli-vada' said for the 10th time? (In the Idli-Vada game, 'idli' is said for multiples of 3 and 'vada' for multiples of 5.)

'Idli-vada' is said for numbers that are multiples of both 3 and 5.

The least common multiple (LCM) of 3 and 5 is 15.

So 'idli-vada' is said at: 15, 30, 45, 60, 75, 90, 105, 120, 135, 150, …

Counting: 1st = 15, 2nd = 30, 3rd = 45, 4th = 60, 5th = 75, 6th = 90, 7th = 105, 8th = 120, 9th = 135, 10th = 150.

Answer: 150

Prime Numbers

We see that 2 is a prime and also an even number. Is there any other even prime?

Any even number greater than 2 can be written as 2 × k for some k > 1.

This means it has 2 and k as factors (in addition to 1 and itself), so it has more than two factors.

Therefore it is composite, not prime.

2 is the only even prime number.

Answer: No, 2 is the only even prime number

Prime Factorisation

Find three prime numbers, all less than 30, whose product is 1955.

1955 ends in 5, so divisible by 5.

1955 ÷ 5 = 391.

Now factorise 391: check primes up to √391 ≈ 19.8.

391 ÷ 7 = 55.8… ✗. 391 ÷ 11 = 35.5… ✗. 391 ÷ 13 = 30.07… ✗. 391 ÷ 17 = 23. ✓

So 391 = 17 × 23.

Check: 5 × 17 × 23 = 5 × 391 = 1955. ✓

All three primes (5, 17, 23) are less than 30. ✓

Answer: 5, 17, and 23

All 42 questions in Prime Time are solved step by step in the Solutions PDF above.

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More solutions in Ganita Prakash

01Patterns in Mathematics02Lines and Angles03Number Play04Data Handling and Presentation06Perimeter and Area07Fractions08Playing with Constructions09Symmetry10The Other Side of Zero
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More NCERT Solutions for Class 6

Read the Prime Time textbook chapter / PDF, or browse all NCERT Class 6 Mathematics solutions.

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