Page 71 - Start Up Mathematics_6

2. The number 17 and ____________ forms a pair of twin primes.

3. There are ____________ pairs of twin primes in the first 100 natural numbers.
4. A number is divisible by ____________ if the number formed by its last two digits is divisible
by it.
5. Two prime numbers are called ____________ primes of n if their average is n.
Answer in One Word or a Line
1. What are the first three multiples of 7?

2. What is the smallest factor of 64?
3. Which is the smallest odd prime number?
4. Are two co-prime numbers always prime? Is the converse true?
5. What is the LCM of two co-prime numbers a and b?

6. Which is greater, HCF or LCM of two numbers? Why?

let’s evaluate

1. Express each of the following numbers as the sum of three odd primes in two different ways:
(a) 43 (b) 61 (c) 83
2. Which of the following pairs of numbers are co-prime?
(a) 16 and 23 (b) 27 and 36 (c) 25 and 64

3. Write the greatest 3-digit number and express it in terms of its prime factors.
4. Is the sum of any two prime numbers always even? Justify your answer with the help of examples.
5. Using divisibility tests, determine which of the following numbers are divisible by 2, 3, 4, 5,
6, 8, 9, 10 and 11 (write Yes or No).

Divisible by
Number
2 3 4 5 6 8 9 10 11
5,444
67,859
5,64,382
9,00,001
8,01,020
8,43,264
6. The students of a class can be divided into groups of 2, 3, 5 or 6. What is the least number
of children this class can have?

7. Find the smallest 6-digit number, which is divisible by 12, 15 and 30.

Thinking Skills

1. If a number is divisible by 2 and 4, is it also divisible by 8?
2. Find two numbers whose LCM is 4 and sum is 5.

3. Find the smallest number which on dividing by 6, 8 and 12 leaves the remainder 4, 6 and 10
respectively.
4. Write all the factors of 4 ?
5

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