# Weekly service

In the class are 20 pupils. How many opportunities have the teacher selected two pupils who will have a week-class service randomly?

Correct result:

n =  190

#### Solution:

$n={C}_{2}\left(20\right)=\left(\genfrac{}{}{0px}{}{20}{2}\right)=\frac{20!}{2!\left(20-2\right)!}=\frac{20\cdot 19}{2\cdot 1}=190$

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