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October 25th, 2017, 08:14 PM  #1 
Newbie Joined: Oct 2017 From: Camboja Posts: 5 Thanks: 0  I need help to calculate a simple problem in probability
Hi folks, i'm not from math field, but i have a question, 1 If in 10 times of event A will occur 2 times event B inside A 2 i know that the chance of event B to occur in sequence is 9, right? x x         1  x x        2   x x       3    x x      4     x x     5      x x    6       x x   7        x x  8         x x 9 3 Then i question, how can i calculate how many times can event B occur NOT in sequence? and how to calculate it? for example  x  x       thanks a lot for any help 
October 25th, 2017, 09:52 PM  #2 
Senior Member Joined: Sep 2015 From: USA Posts: 2,100 Thanks: 1093 
There are $\dbinom{10}{2}=45$ ways that 2 x's can occur out of the 10 events. You showed there are 9 ways that the two x's can occur sequentially. Thus there are 459=36 ways the two x's can occur nonsequentially. Last edited by romsek; October 25th, 2017 at 09:55 PM. 
October 25th, 2017, 09:59 PM  #3 
Newbie Joined: Oct 2017 From: Camboja Posts: 5 Thanks: 0 
thanks for answering, i was thinking it was way more thank 100 possibilities, have a nice day


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