You are Here: Home >< Maths

# Statistics normal distribution mean and variance watch

1. A study showed that the time, T minutes, spent by a customer between entering and leaving Fely's department store has a mean of 20 with a standard deviation of 6.
Assume that T may be modelled by a normal distribution.
a.) find the value of T exceeded by 20% of customers
b.) i.) write down the standard deviation of the mean time spent in Fely's store by a random sample of 90 customers
ii.) find the probablilty that this mean time will exceed 21 minutes
2. (Original post by Kmarsden98)
A study showed that the time, T minutes, spent by a customer between entering and leaving Fely's department store has a mean of 20 with a standard deviation of 6.
Assume that T may be modelled by a normal distribution.
a.) find the value of T exceeded by 20% of customers
b.) i.) write down the standard deviation of the mean time spent in Fely's store by a random sample of 90 customers
ii.) find the probablilty that this mean time will exceed 21 minutes
Part A)

Go to percentage points table, look for 0.2000 read off the z value, then use the formula making it equal to that Z value, so it'd be

T-20/6 = 0.8416
t-20 = 5.0496
T=25.0496

Is this right?
3. Moved to maths
4. (Original post by iMacJack)
Part A)

Go to percentage points table, look for 0.2000 read off the z value, then use the formula making it equal to that Z value, so it'd be

T-20/6 = 0.8416
t-20 = 5.0496
T=25.0496

Is this right?
Unless all my stats knowledge has deserted me since last year (perfectly possible tbh) than it looks right to me
5. (Original post by Kmarsden98)
A study showed that the time, T minutes, spent by a customer between entering and leaving Fely's department store has a mean of 20 with a standard deviation of 6.
Assume that T may be modelled by a normal distribution.
a.) find the value of T exceeded by 20% of customers
b.) i.) write down the standard deviation of the mean time spent in Fely's store by a random sample of 90 customers
ii.) find the probablilty that this mean time will exceed 21 minutes
Have you tried drawing a sketch of the bell curve and marking what you need on it? You should try using the symmetry of the curve and have a look at the statistical tables.
Spoiler:
Show
Try using the percentage point table.

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: February 14, 2016
The home of Results and Clearing

### 1,395

people online now

### 1,567,000

students helped last year
Today on TSR

### University open days

1. Keele University
Sun, 19 Aug '18
2. University of Melbourne
Sun, 19 Aug '18
3. Sheffield Hallam University
Tue, 21 Aug '18
Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams