The pair will be 15 - 16, 16 - 21, 21 - 28, 15 - 28 and 16 - 28
The HCF of 15 and 16
HCF of 16 and 21
HCF of 15 and 28
From above it is clear that 15 and 16 are co-prime because the common factor is 1
Hence pairs 15 and 16, 16, 21, 15, 28 are a co-prime number.
