destroyer quant 64

[pistolpete007]

im sure its not an error but it seems like it

the question is the weatherman tells u that there is a 70% chance of rain each day for the next 3 days. Whst is the probability it will 2 out of the 3 days.

The formula to use is nCr p^n(1-p)^r
-where p=the probabilty of an event occuring
-n=total number of trials
-r=number of trials having the desired results

destroyer did (3!/(2!1!)(7/10)^2 (.3)^1
the part in the red is what i dont agree with there were 3 total trials so shuldnt n=3 and the desired we wanted 2 rainy days so shouldnt r=1

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[Tina324]

Hi pistolpete007,

nope, from my understanding the answer is correct. I hate formulas, so I'll explain it the way I do it, and hopefully you can take what you need from that.

(7/10)*(7/10)*(3/10)*3=.441=44%

We want to know the probability that it will rain for 2 days. Those are 2 separate events, so n should equal 2. We also have to take care of the one other day in which we don't want it to rain (3/10)...therefore r should equal 1.

And the reason we multiply the whole expression by 3 (which you already understand, but I'll explain anyways) is because the 2 days in which it can rain can happen in 3 different ways...
it may rain on day1 and day2...or...day1 and day3..or...day2 and day3..

Hope that helps!

 

[Streetwolf]

im sure its not an error but it seems like it

the question is the weatherman tells u that there is a 70% chance of rain each day for the next 3 days. Whst is the probability it will 2 out of the 3 days.

The formula to use is nCr p^n(1-p)^r
-where p=the probabilty of an event occuring
-n=total number of trials
-r=number of trials having the desired results

destroyer did (3!/(2!1!)(7/10)^2 (.3)^1
the part in the red is what i dont agree with there were 3 total trials so shuldnt n=3 and the desired we wanted 2 rainy days so shouldnt r=1

No, r = 2. The formula you have is wrong. It should have these terms:

p^r (this is the probability of the event occurring, and the power is the # of times you want it to happen)

(1-p)^(n-r) (this is the probability of the event NOT occurring, and the power is the # of times it will NOT happen)