which of the following is a pair of co - primes (a) 20 , 25 (b) 18 , 35 (c) 15 , 63 (d) 28 , 81
Answers
Answer:
18 and 35 are a pair of co- prime numbers. That is option B.
Answer:
b.) 18, 35 and d.) 28, 81
Step-by-step explanation:
Co-primes are such numbers whose HCF is 1.
Our first step should always be to look for primes in the options. If there is one and the other number is not the prime's multiple, they will always be co-primes.
As there are no primes in the options, we resort to finding their HCFs.
a.) HCF of 20 and 25:
25 = 20*1 + 5
=> 20 = 5*4 + 0
Thus, 20 and 25 are not co-primes as their HCF is 5.
b.) HCF of 18 and 35:
35 = 18*1 + 17
=> 18 = 17*1 + 1
=> 17 = 1*17 + 0
Thus, 18 and 35 are co-primes as their HCF is 1.
c.) HCF of 15 and 63:
63 = 15*4 + 3
=> 15 = 3*5 + 0
Thus, 15 and 63 are not co-primes as their HCF is 3.
d.) HCF of 28 and 81:
81 = 28*2 + 25
=> 28 = 25*1 + 3
=> 25 = 3*8 + 1
=> 3 = 1*3 + 0
Thus, 28 and 81 are co-primes as their HCF of 1.