Thread: A Level Normal Distribution question

Hi, I've been looking at this question for nearly an hournow and I can't figure out what to do, I've looked through my notes and my textbook and nothing helps so could someone please talk me through what I need to do?


Question:
1. An athlete believes that her times for running 200 metres in races are normally distributed
with a mean of 22.8 seconds.
(a) Given that her time is over 23.3 seconds in 20% of her races, calculate the variance of
her times.
[5 marks]
(b) The record over this distance for women at her club is 21.82 seconds. According to her
model, what is the chance that she will beat this record in her next race?
[3 marks]

Thanks so much in advance

Look at this thread.

RonL

Thanks

Where do you get the 0.84 from?

Originally Posted by jonannekeke

Thanks

Where do you get the 0.84 from?

Table look-up in the table of the cumulative standard normal distribution.

For the standard normal P(z<0.84)~=0.8, so P(z>0.84) ~= 0.2

RonL

I know I sound really stupid but I sill don't understand


Oooo never mind, I just found the answer to this question in my book, we did this question ages ago as practice but thanks for your help anyway