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Question Number 160389 by mr W last updated on 28/Nov/21

Commented by mr W last updated on 29/Nov/21

$t h a n k s f o r t r y i n g!$ $b u t {i t}^{'} s w r o n g s i r . i t h i n k y o u$ $m i s u n d e r s t o o d t h e q u e s t i o n . i t i s n o t$ $t o s e l e c t o n e p a i r f r o m 20 p e o p l e, b u t$ $t o b u i l d 10 p a i r s f r o m 20 p e o p l e .$ $t h e t w o s t u d e n t s f r o m a n y c o l l e g e$ $s h o u l d p l a y a g a i n s t t h e s t u d e n t s$ $f r o m o t h e r c o l l e g e s, n o t a g a i n s t e a c h$ $o t h e r, s o t h e y s h o u l d n o t b e i n a p a i r .$
Commented by cortano last updated on 29/Nov/21

$n (S) = C_{2}^{20} = 190$ $n (A) = 190 - 10 = 180$ $P (A) = \frac{180}{190} = \frac{18}{19}$
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