(Class X)

       Exercise 1.2


Question 1:

Express each number as product of its prime factors:

(i) 140 (ii) 156 (iii) 3825 (iv) 5005 (v) 7429

Answer 1:



Question 2:

Find the LCM and HCF of the following pairs of integers and verify that
LCM × HCF = product of the two numbers.

(i) 26 and 91 (ii) 510 and 92 (iii) 336 and 54

Answer 2:


Hence, product of two numbers = HCF × LCM



Hence, product of two numbers = HCF × LCM


Hence, product of two numbers = HCF × LCM


Question 3:

Find the LCM and HCF of the following integers by applying the prime
factorisation method.

(i) 12, 15 and 21 (ii) 17, 23 and 29 (iii) 8, 9 and 25

Answer 3:








Question 4:

Given that HCF (306, 657) = 9, find LCM (306, 657).

Answer 4:



Question 5:

Check whether 6n can end with the digit 0 for any natural number n.

Answer 5:

If any number ends with the digit 0, it should be divisible by 10 or in
other words, it will also be divisible by 2 and 5 as 10 = 2 × 5

Prime factorisation of 6n = (2 ×3)n

It can be observed that 5 is not in the prime factorisation of 6n.
Hence, for any value of n, 6n will not be divisible by 5.

Therefore, 6n cannot end with the digit 0 for any natural number n.


Question 6:

Explain why 7 × 11 × 13 + 13 and 7 × 6 × 5 × 4 × 3 × 2 × 1 + 5 are
composite numbers.

Answer 6:

Numbers are of two types - prime and composite. Prime numbers can be
divided by 1 and only itself, whereas composite numbers have factors
other than 1 and itself.

It can be observed that

7 × 11 × 13 + 13 = 13 × (7 × 11 + 1) = 13 × (77 + 1)

= 13 × 78

= 13 ×13 × 6

The given expression has 6 and 13 as its factors. Therefore, it is a
composite number.

7 × 6 × 5 × 4 × 3 × 2 × 1 + 5 = 5 ×(7 × 6 × 4 × 3 × 2 × 1 + 1)

= 5 × (1008 + 1)

= 5 ×1009

1009 cannot be factorised further. Therefore, the given expression has 5
and 1009 as its factors. Hence, it is a composite number.



Question 7:

There is a circular path around a sports field. Sonia takes 18 minutes to
drive one round of the field, while Ravi takes 12 minutes for the same.
Suppose they both start at the same point and at the same time, and go
in the same direction. After how many minutes will they meet again at
the starting point?

Answer 7:

It can be observed that Ravi takes lesser time than Sonia for completing
1 round of the circular path. As they are going in the same direction, they
will meet again at the same time when Ravi will have completed 1 round

of that circular path with respect to Sonia. And the total time taken for
completing this 1 round of circular path will be the LCM of time taken by
Sonia and Ravi for completing 1 round of circular path respectively i.e.,
LCM of 18 minutes and 12 minutes.

18 = 2 × 3 × 3

And, 12 = 2 × 2 × 3

LCM of 12 and 18 = 2 × 2 × 3 × 3 = 36

Therefore, Ravi and Sonia will meet together at the starting point after
36 minutes.