Since the total number of students is 21
Since they will work in pairs, that means each group has 2 students
So it is like picking up 2 students from 21 students
That means we will want how many combinations we can do
Then
n = 21
r = 2
We will use the rule of combination
[tex]^nC_r=\frac{n!}{(n-r)!.r!}[/tex]Substitute the values of n and r in the rule
[tex]\begin{gathered} ^{21}C_2=\frac{21!}{(21-2)!.2!} \\ \\ ^{21}C_2=210 \end{gathered}[/tex]There are 210 introductions were made
