the lengths in cm of drinking straws produced in a factory have a normal distribution with mean mu and variance 0.64. it is given that 10% of the straws are shorter than 20cm
(i) find the value of mu.
It says you first find the s.d. which is sqrt:0.64
which is 0.8.
then with the formula
you place \[ 20- mu \over 0.8\].Then it says
<10% or >90%. The mark scheme uses z>90% so it ends up as (p) = -1.282 according to the normal distribution chart. So, you equate that formula to that, but why can't it be <10%?

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To use <10% you need a -z z table|dw:1337763325038:dw|

This doesn't help...

z = -1.282\[-1.282=\frac{20-\mu}{0.8}\]
\[-1.0256=20-\mu\]
\[\mu=21.0256cm\]

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