Check whether the following number is odd number or not: $21$

Verify the distributive property for multiplication of the following:

$a\times \left(b+c\right)$, where $a= 28, b= 15, c=12$.

Verify the distributive property for multiplication of the following:

$a\times \left(b-c\right)$, where $a= 28, b= 15, c=12$

Check whether the following pair of numbers is co-prime.

$20, 25$

Check whether the following pair of numbers is co-prime.

$18, 35$

Express the following number as the sum of two odd primes:

$80$

Express the following number as the sum of two odd primes:

$100$