Prime Time

At what number is 'idli-vada' said for the 10th time?

If the game is played for the numbers from 1 till 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'?

What if the game was played till 900? How would your answers change?

Find all multiples of 40 that lie between 310 and 410.

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.

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.

Find the common factors of: a. 20 and 28 b. 35 and 50 c. 4, 8 and 12 d. 5, 15 and 25

Find any three numbers that are multiples of 25 but not multiples of 50.

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'?

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

Find the smallest number that is a multiple of all the numbers from 1 to 10.

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

Which of the following numbers are prime? 23, 51, 37, 26

Write three pairs of prime numbers less than 20 whose sum is a multiple of 5.

The numbers 13 and 31 are prime numbers. Both these numbers have same digits 1 and 3. Find such pairs of prime numbers up to 100.

Find seven consecutive composite numbers between 1 and 100.

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.

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.

Which of the following numbers is the product of exactly three distinct prime numbers: 45, 60, 91, 105, 330?

How many three-digit prime numbers can you make using each of 2, 4 and 5 once?