Stuck on a few A-Level Stats questions-help please!!!

[zkm1223]

Hi, i am stuck on a few a-level stats questions and need help on them ASAP. here they are:

1) A woman orders a new freezer from a well known supplier. She is told that it will arrive on a particular day between 07.00 and 18.00, but not between 13.00 and 15.00 because that's when the delivery driver takes his lunch. When she asked if they could be more specific, she was told that it was complelety at random. Let X be the number of hours after 06.00 when the freezer actually arrives.

a) Sketch the probability density function f(x).
b) Hence state f(x) in algebraic form.
c) Calculate the standard deviation of X.
d) Calculate the inter-quartile range of X.

2) Low energy light bulbs have an expected lifetime of 3000 operating hours with a SD of 550 operating hours. Assuming that lifetimes follow a Normal distribution, calculate:

a) the probability that the bulb lasts more than 2100 hours
b) the conditional probability that a bulb which has already lasted for 2500 hours will not fail before 4600 hours
c) the operating time by which 96% of bulbs in use would have failed.

3) A group of students conducted a survey of login times in different classrooms and obtained the following results:


Room A (Mean) 84 (SD) 58
Room B (Mean) 59 (SD) 32

The teacher was suspicious and decided to check these results by logging in to 7 computers in each room.
Assuming that the students' results were correct and that login times are independently Normally distributed:

a) what is the probability that the average of his login times in Room A was less than 59 seconds?
b) What is the probability that the average of his login times in Room A was more than twice the average login times in Room B?
c) what evidence is there that the login times were not Normally Distributed?


Please can you explain these to me. I have looked in text books but I cannot understand how to do these. thanks in advance.

[jemidiah]

Disclaimer: I'm not trying to be mean, I'm just saying what I think will help you the most.

If you can't do any of those problems, a forum post won't be enough to help you. It would be pages long, and you've already said that the textbook doesn't help, so most likely you wouldn't get that much from another text-based explanation. This material is much better explained person-to-person as well, in my opinion. I'd highly recommend that you either get help from your teacher or find a tutor (possibly just a friend who's good at stats).

If you narrowed the scope of your question to one or maybe two of those sub parts, then things would be different. As it stands, though, it's just too many concepts to adequately explain. Sorry .

[zkm1223]

These are some questions from the exercises in the textbook which I don't understand. I have looked at the examples and I can't understand these specific questions from the examples.

The main question I am stuck on is question 1. I have managed to do some of question 2 and I sort of understand question 3 but am completely stuck on question 1 so if someone could help me on question 1 that will be very helpful. thanks again.

[sparbag]

hello,

Sorry for dragging this post back but having been helping my daughter through these last term I understand the frustration of the student.

Please see the links. One is for all of the tutorials and the second is directly to probability density function.

http://www.examsolutions.co.uk/A-Lev...ials/index.php

http://www.examsolutions.co.uk/maths...tutorial-1.php
Kind regards

Steve