CO-PRIME NUMBERS WORKSHEET

Find which of the following pairs of numbers are co-primes. 

1)  4 and 12.

2)  7 and 43. 

3)  5 and 3. 

4)  8 and 17. 

5)  8 and 15. 

6)  14 and 21. 

7)  2 and 4. 

8)  1 and 2. 

9)  If the product of two co-prime numbers is 345, find the LCM of the numbers. 

10)  The LCM of two co-prime numbers is 420. If one of the numbers is 15, then find the other number.

Answer (1) : 

4 and 12 have the common divisors other than 1.

They are 2 and 4. 

So, 4 and 12 are not co-primes. 

Answer (2)

7 and 43 have no common divisor other than 1.  

So, 7 and 43 are co-primes. 

Answer (3) :

5 and 3 have no common divisor other than 1.  

So, 5 and 3 are co-primes. 

Answer (4) :

8 and 17 have no common divisor other than 1.  

So, 8 and 17 are co-primes. 

Answer (5) :

8 and 15 have no common divisor other than 1.   

So, 8 and 15 are co-primes. 

Answer (6) :

14 and 21 have common divisor other than 1.

That is 7.

So, 14 and 21 are not co-primes. 

Answer (7) :

2 and 4 have common divisor other than 1.

That is 2.   

So, 2 and 4 are not co-primes. 

Answer (8) :

1 and 2 have no common divisor other than 1.   

So, 1 and 2 are co-primes. 

Answer (9) :

Because the two numbers are co-prime, their HCF is 1. 

We know that,

LCM x HCF  =  Product of the two numbers 

Substitute. 

LCM x 1  =  345

LCM  =  345

Answer (10) :

Let x be the other number.

We know that,

Product of the two numbers  =  LCM x HCF

As the HCF of co-primes is 1,

15(x)  =  420 x 1

15x  =  420

Divide each side by 15.

x  =  28

The other number is 28.

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