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 answer:

n =  190

Step-by-step explanation:

$n=C_2(20)=\left({\displaystyle \genfrac{}{}{0px}{}{20}{2}}\right)={\displaystyle \frac{20!}{2!(20-2)!}}={\displaystyle \frac{20\cdot 19}{2\cdot 1}}=190$

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