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Statistics normal distribution mean and variance watch

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    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
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    (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?
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    Moved to maths
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    (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
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    (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.
 
 
 
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