another probability problem!

• Aug 18th 2011, 03:04 AM
iamamathnoob
another probability problem!
From the past studies it is know that 60% of the students at HIGH read the Age newspaper. There was a recent article in the Age on the rise in living costs in Melbourne. If the class size is 20,

A) what is the probability that none of the students read the article?
B) what is the probability that at least 75% of the students read the article?

Do I use the bionomial probability?

I am hopeless at math. Any hints or input would be muchly appreciated.
• Aug 18th 2011, 03:33 AM
Siron
Re: another probability problem!
Yes, the binomial contribution is fine.
Can you now solve the problem? ...
• Aug 18th 2011, 03:54 AM
iamamathnoob
Re: another probability problem!
Its nice to know i am on the right track! :)
i was taught calculating binomial using the nCr calculator function today. Im just still unsure of what the terms p and n is...
the 60% really throws me off and gets me confused...
• Aug 18th 2011, 04:02 AM
Siron
Re: another probability problem!
The best way to recognize if you're dealing with a binomial distribution (it has to be distribution not contribution, sorry for that) is to see if you've success or not, in this case you've success if they read the article and no success if they don't so it's a binomial distribution.

Use: p=0,6 and n=20
• Aug 18th 2011, 04:07 AM
iamamathnoob
Re: another probability problem!
ill be sure to keep that in mind.

= 1.0995 ?

fingers crossed its correct.
• Aug 18th 2011, 04:13 AM
Siron
Re: another probability problem!
Is this the answer to question a)?
Can you show me how you've calculated it, because honestly I've no calculator over here :).
For question b) Have you any idea how to solve that one?
• Aug 18th 2011, 04:23 AM
iamamathnoob
Re: another probability problem!
P(X=0): 20C0 * 0.6^0 * 0.4^20-0 = 1.0995

and still no clue for question b)
>.<
• Aug 18th 2011, 06:26 AM
Siron
Re: another probability problem!
a) Check your calculation again, a probability P can never be >100%.
b) What's 75% of 20? Important here is the 'at least' that means 75% or more so ...
• Aug 18th 2011, 11:32 AM
HallsofIvy
Re: another probability problem!
Quote:

Originally Posted by iamamathnoob
P(X=0): 20C0 * 0.6^0 * 0.4^20-0 = 1.0995

No. 20C0= 1 (nC0= 1 for all n), 0.6^0= 1 (any number to the 0 power is 1) and 0.4^20 is a very small number.

Quote:

and still no clue for question b)
>.<
"At least 75%" means "at least 15". You will need to calculate the probability for 15, 16, 17, 18, 19, and 20, and add.
• Aug 18th 2011, 01:30 PM
iamamathnoob
Re: another probability problem!
yeah that's the idea i had for question b) to calculate the probability for 15, 16, 17, 18, 19, and 20, and add...

thanks guys, you have been a very big help =)
• Aug 19th 2011, 02:31 AM
Siron
Re: another probability problem!
You're welcome! And indeed these 'little words' like: at least, at most, ... are really important to understand the problem.