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# probability basics... need help

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probability basics... need help [#permalink]  03 Jun 2006, 08:46
hi... i jus don seem to grasp the concept... if any one could kindly help me out here pleaseeeeeeee...
give me basics and

explain dis prob

if 4 dice are thrown simultaneously,what is the probability tat the sum of the numbers is exactly 20

{6,6,6,2} = 4!/3! = 4

{6,6,5,3} = 4!/ 2!
= 4

{6,6,4,4} = 4! /2!2! = 6

{6,5,5,4} = 4! / 2! = 12

{5,5,5,5} = 4!/4! = 1

ans = 35/1296

wat do these exclamatory s denote??? hz it 4!/2! 2! = 6???? please explain the whole problem
Manager
Joined: 19 Apr 2006
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the (!) = factorial

see this site, it explains it well
http://mathworld.wolfram.com/Factorial.html
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Re: probability basics... need help [#permalink]  03 Jun 2006, 19:57
yasmeen wrote:
hi... i jus don seem to grasp the concept... if any one could kindly help me out here pleaseeeeeeee...
give me basics and

explain dis prob

if 4 dice are thrown simultaneously,what is the probability tat the sum of the numbers is exactly 20

{6,6,6,2} = 4!/3! = 4

{6,6,5,3} = 4!/ 2!
= 4

{6,6,4,4} = 4! /2!2! = 6

{6,5,5,4} = 4! / 2! = 12

{5,5,5,5} = 4!/4! = 1

ans = 35/1296

wat do these exclamatory s denote??? hz it 4!/2! 2! = 6???? please explain the whole problem

To calculate this problem, first consider the total number of ways 4 dices can be thrown. There's 6 possibilities for the first dice (1, 2, 3, 4, 5, 6), 6 possibilities for the second and so on. Therefore, there are 6x6x6x6 ways for the dices to land = 1296.

Now you need to figure out the number of ways to throw a perfect 20. {6,6,6,2} denotes a way to throw a perfect 20. To calculate the number of ways you can throw this combination of numbers, use the formula: n!/r! where r represents the number of repetitions in your group and n is the number of units. Therefore, you get 4!/3! = 4.

ie)
6.6.6.2
6.6.2.6
6.2.6.6
2.6.6.6

Do this same calculation for all your other options and you'll get 35, the total number of ways to throw 20. Put that over the total amount and you''ll find the probability.

Note: I think you made a mistake here

{6,6,5,3} = 4!/ 2!
= 4

This should equal 12 since 4!/2! = 4x3 = 12.
Re: probability basics... need help   [#permalink] 03 Jun 2006, 19:57
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