Problem 1

Probability Level 1

In how many ways can two prefects be selected from a class of 20 if the order doesn't​ matter?

Check out the set: Combinatorics .
190 10 40 400

Angela Fajardo by Angela Fajardo , 19, Philippines

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1 solution

Zee Ell
Aug 29, 2016

We can select the first prefect 20 ways and the second prefect 19 ways. Since the order doesn't matter, we have to divide our result by 2.

20 × 19 2 = 190 \frac {20 × 19}{2} =\boxed {190}

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