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manigraja100
Students
 
Posts: 14
Joined: Sun Mar 03, 2013 12:00 am
 

word problem.pg-106

by manigraja100 Mon Jun 24, 2013 12:05 am

Hi Tommy,
I saw your explanation about the slot method for counting. You said "If order doesn't matter then multiply all slots together,then divide by the # of slots factorial wherever order doesn't matter". I didn't understand the denominator part. for e.g " If three of seven standby passengers are selected for a flight,how many different combinations of standby passengers can be selected? " The numerator will be 7*6*5, what will be the denominator.Please explain.
P.S. I can solve this using combination formula,but i want the slot method explanation.
tommywallach
Manhattan Prep Staff
 
Posts: 1917
Joined: Thu Mar 31, 2011 11:18 am
 

Re: word problem.pg-106

by tommywallach Wed Jun 26, 2013 6:56 pm

Hey Mani,

Order doesn't matter (because person 1, person 2, person 3 is the same GROUP as person 2, person 3, person 1), so we divide by the # of slots factorial. In this case, that's 3!.

-t

P.S. As I said in your previous combinatorics question, I always use the slots method. It's more flexible/conceptual.