Question

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%?

Answer 1

To use <10% you need a -z z table