Discrete Mathematics How to calculate subset 20 how does thi

Discrete Mathematics

How to calculate subset:
(20)   how does this equal 4845?
( 4 )

Also how to multiply:

c(8, 2) * c(12,2) =1848 how is this 1848?

Solution

first part , problem not clear .

C(n,r) =n C r = n! / (n-r)!* r!

there fore c(8 ,2)=28

c(12 ,2)= 66

so c(8, 2) * c(12,2) =1848