# working out probabiblities (help)

• Feb 15th 2008, 02:12 AM
pitt
working out probabiblities (help)
A computer games retailer knows that there is a 62% chance that a customer coming into its store will buy a game out of the top ten games chart. 8 individual customers visit the shop in one hour.

(i) What is the probability that exactly 6 of them will buy a game from the top ten chart?
(3 marks)

(ii) What is the probability that no more than 7 will buy a game from the top ten chart?

can someone show me how to do this?? im preparing for my exams and need to understand this (im looking at past exams). And if possible could you recommend me a site to understand this? thanks
• Feb 15th 2008, 03:02 AM
mr fantastic
Quote:

Originally Posted by pitt
A computer games retailer knows that there is a 62% chance that a customer coming into its store will buy a game out of the top ten games chart. 8 individual customers visit the shop in one hour.

(i) What is the probability that exactly 6 of them will buy a game from the top ten chart?
(3 marks)

(ii) What is the probability that no more than 7 will buy a game from the top ten chart?

can someone show me how to do this?? im preparing for my exams and need to understand this (im looking at past exams). And if possible could you recommend me a site to understand this? thanks

First try following the three-prong approach. Then, if you're still stuck, say where you're stuck.
• Feb 15th 2008, 06:53 AM
pitt
n=8

p=0.6

and now??

im really lost, could you just do the calculations?? i think it will be easier for me to understand, im really bad with maths
• Feb 15th 2008, 01:19 PM
mr fantastic
Quote:

Originally Posted by pitt
n=8

p=0.6 Mr F says: No, it's 0.62. That's just too careless.

and now??

im really lost, could you just do the calculations?? i think it will be easier for me to understand, im really bad with maths

Sorry, but I'm not going to do that.

I would like to see you try to set this problem out line by line - carefully following the process outlined in the example I refered you to. Show me all the steps. If there's a particular part you're stuck on, I will help you through that part.

It's not good enough to say 'I'm really bad at maths' and use that as an excuse for not trying to work through the process ..... What is step 1? Show it to me. What is step 2? etc. Think about what you're trying to do.

I realise this involves some pain at your end - the pain is for a greater good.
• Feb 19th 2008, 09:11 AM
pitt
tried everything but still cant understand it, seems like there are different rules for every calculation, that makes it hard to understand and i dont understand why there are so many numbers on brackets (on the example you showed me)
• Feb 19th 2008, 03:17 PM
mr fantastic
Quote:

Originally Posted by pitt
A computer games retailer knows that there is a 62% chance that a customer coming into its store will buy a game out of the top ten games chart. 8 individual customers visit the shop in one hour.

(i) What is the probability that exactly 6 of them will buy a game from the top ten chart?
(3 marks)

(ii) What is the probability that no more than 7 will buy a game from the top ten chart?

can someone show me how to do this?? im preparing for my exams and need to understand this (im looking at past exams). And if possible could you recommend me a site to understand this? thanks

The three-prong approach, sport:

1. Define the random variable:

Let X be random variable number of customers that buy a game out of the top ten games chart in one hour.

2. Define the distribution followed by the random variable:

X ~ Binomial(n = 8, p = 0.62).

It's binomial because the three criteria are satisfied:
* There are two possible outcomes - buy a game in the top ten ('success') or not buy a game iin the top ten ('failure').
* Each trial (the game a customer buys) is independent.
* The probability of 'success' (buying a game in the top ten) in each trial stays the same.

The number of trials is 8 (8 customers in one hour) so n = 8.

The probability of success in a single trial (a customer buying a game in the top ten) is 0.62 (62% chance) so p = 0.62.

3. Write a probability statement of the problem:

(i) Pr(X = 6) = ?

(ii) Pr(X < 8) = 1 - Pr(X = 8) = ?

There are several ways to do these calculations:

(a) Use technology eg. use TI-83 to find

(i) binompdf(8, 0.62, 6).
(ii) 1 - binompdf(8, 0.62, 8).

(b) Use the formula $Pr(X = r) = {n \choose r} \, p^r \, (1 - p)^{n-r} = {8 \choose r} (0.62)^r (1 - 0.62)^{8-r} = {8 \choose r} \, (0.62)^r \, (0.38)^{8-r}$ to calculate
(i) Pr(X = 6) (use r = 6).
(ii) Pr(X = 8) (use r = 8). Then get 1 - Pr(X = 8).

You must be familiar with this formula and how to use it ......

The calculations are left for you to do.
• Feb 19th 2008, 03:25 PM
mr fantastic
Quote:

Originally Posted by pitt
tried everything but still cant understand it, seems like there are different rules for every calculation, that makes it hard to understand and i dont understand why there are so many numbers on brackets (on the example you showed me)

There is a basic process that is used every single time. The only things that change are the what the random variable is and what values of it you want, the values of n and p. These things are mere details in the basic process.

You will surely notice that post #6 is nothing more than a cut and paste of the link I provided - only the details got changed ......
• Feb 20th 2008, 04:25 AM
pitt
someone done for me the question in another forum, thanks for your help anyway, i know you wanted me to learn and thats why you told me to do it but i really couldnt understand much and i still cant, but im going to ask my tutor next week to show me how it works (now i just dont understand how to use the calculator bit)..
im looking at past exam papers and working on them so i get a pass in the exam in may
• Feb 20th 2008, 04:57 AM
bobak
Quote:

Originally Posted by pitt
someone done for me the question in another forum, thanks for your help anyway, i know you wanted me to learn and thats why you told me to do it but i really couldnt understand much and i still cant, but im going to ask my tutor next week to show me how it works (now i just dont understand how to use the calculator bit)..
im looking at past exam papers and working on them so i get a pass in the exam in may

You really shouldn't post a question to two forums, You just now wasted Mr F's time. People choose to give up their time to help you don't abuse it by making their efforts redundant.
• Feb 20th 2008, 08:55 AM
topsquark
Quote:

Originally Posted by bobak
You really shouldn't post a question to two forums, You just now wasted Mr F's time. People choose to give up their time to help you don't abuse it by making their efforts redundant.

Unless I am missing something, he apparently posted it on another site.

-Dan