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    Question taken from S1 jan 2008 AQA.
    Q)1)b) Sawmill requires batch of logs such that there is a probability of 0.025 that any given log will have a length less than 3.1. determine the value of mu/mean. We also know that the SD is 0.16.
    When i look up the Z value for 0.025 i get 1.96.
    However on the mark scheme when its subbed into the equation it gives z = 3.1-μ/0.16 = -1.96 i dont understand why it becomes negative? does anyone have an answer to why??
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    (Original post by redmcq)
    Question taken from S1 jan 2008 AQA.
    Q)1)b) Sawmill requires batch of logs such that there is a probability of 0.025 that any given log will have a length less than 3.1. determine the value of mu/mean. We also know that the SD is 0.16.
    When i look up the Z value for 0.025 i get 1.96.
    However on the mark scheme when its subbed into the equation it gives z = 3.1-μ/0.16 = -1.96 i dont understand why it becomes negative? does anyone have an answer to why??
    Presumably you're looking at the percentage points table which clearly gives \mathbb{P}(Z > 1.96) = 0.025.

    What you want here is \mathbb{P}(Z < z) = 0.025, in which case, you need to get this to:

    \mathbb{P}(Z > -z) = 0.025 which lets you state that -z = 1.96 right away.

    Watch your inequality signs.
 
 
 
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