A hint to get started
Convert both rejection rates into quantile equations
A grading machine rejects apples as too large if their mass exceeds 150 g, and too small if below 110 g. Over a long period 10% are too large and 15% too small. Apple mass in grams may be modelled by . Find μ and σ.
[2005/Specimen Paper/II/9(modified)]
CJC · Discussion · Question 4
A10% upper tail is the 90 th percentile, not the 10 th.