S2 probability question (don't have the answers)

A balloon manufacturer claims that 95% of his balloons will not burst when blown up.
If you have 20 of these balloons to blow up for a birthday party

(a) What is the probability that none of them will burst when blown up?
(0.95^20)(0.05^20) <--- is this right? looks too simple

(b) Find the probability that exactly 2 balloons burst.
(20!/2!)(0.05^2)(0.95^18)

are these answers correct?

Reply 1

Adam92

This is a simple binomial distribution, with p = 0.95, n = 20.

Just use the binomial formula?

(Ps. no they are not correct)

(edited 14 years ago)

Reply 2

doin' your mum

and i also don't know how to work out P(X =< 7) from the random variable X - B(20,0.4). wtf is a cumulative binomial expansion distribution table? is it that long table at the back of the S1 formula sheet? rofl

Reply 3

Adam92

& yes, use the table!

Reply 4

doin' your mum

Adam92

This is a simple binomial distribution, with p = 0.95, n = 20.

Just use the binomial formula?

(Ps. no they are not correct)

eh, i thought i used the formula...

how should it be done then?

Reply 5

anshul95

no you have not used the binomial formula properly and no it isn't that table at the back of S1 its in the S2 section of the formula book.
PS: For the first one think how you would do it if you didn't have the binomial formula (remember they are independent events). You should see where you are going wrong.

(edited 14 years ago)

Reply 6

Adam92

doin' your mum

eh, i thought i used the formula...

how should it be done then?

Nope you didn't.

P(X=x) = (nCx)(p)^x(1-p)^(n-x)

(edited 14 years ago)

Reply 7

doin' your mum

k so can someone just tell me how to do it? lol...

Reply 8

Adam92

doin' your mum

k so can someone just tell me how to do it? lol...

Not allowed :rolleyes:

Just put the numbers into the formula I just gave you..

Reply 9

doin' your mum

Adam92

Not allowed :rolleyes:

Just put the numbers into the formula I just gave you..

why would it not be allowed? i learn better from examining solutions as opposed to being told how to do something which i thought i already did...

(20!/2!)(0.05^2)(0.95^18)

how is this not the formula u just gave me?