标题: Probability [打印本页] 作者: busprof 时间: 2011-7-13 14:57 标题: Probability
Friends lil help required with probablity.
Suppose i have a 80% Free throw percentage and i have 5 chances out of which i have to make 3 baskets.
I am trying to solve this by 2 different methods, getting correct answer from method 1 but not from second method.
Method 1.
5 nCr 3 = 10 Ways.
(Probability Of Success) * (Probability Of Failure) * Total ways
(.80 * .80 * .80) * (.20 * .20 ) * 10
= 20.480%
Method 2
Prob (a) = event : a true / total # of events
= 10 / 32
= 31.25%
-where total events are 2*2*2*2*2 = 32.
i dont know whats wrong with method 2 or maybe em missing something.作者: studyn 时间: 2011-7-13 14:57
Ok, so in case 1, the way the formula works is that it first establishes that you made 3 throws and lost 2 throws. Hence (.80 * .80 * .80) * (.20 * .20 ).
Then it accounts the fact that you could have made the three throws in 10 different orders. So, it's (.80 * .80 * .80) * (.20 * .20 )*10.
What you have done in case 2 is equivalent to:
0.5*0.5*0.5*0.5*0.5*10.
In other words, case 2 assumes that your probability of making each throw is 50%, not 80% as stated in the question.
ALSO:
The case 1 method does not give you the probability of making at least 3 throws. It gives you the probability of making exactly 3 throws. I assume that you want this latter thing, since you're saying your answer was correct.作者: Jolyn 时间: 2011-7-13 14:57
as others have pointed out, the first method is correct if you want to score exactly 3 baskets. If you want to score at least 3 baskets, you need to add probability of scoring 4 and 5. You will get 94.21%.作者: soddy1979 时间: 2011-7-13 14:58
So do you guys mean that this question cannot be solved from 2nd method? if so pl tell me how.
by the way thanks everyone for your replies.作者: joemoran 时间: 2011-7-13 14:58
ohai Wrote:
-------------------------------------------------------
> Huh what? I just explained what the second method
> is doing.
I absolutely agree with that explanation. Method 2 assumes that all outcomes have equal probabilities which is true when p = 1/2.
