Weights of apples in a packhouse: normal(µ=180g, σ=10g)

The diagrams on this page describe two ways to find quantiles for a normal distribution. Consider the following question:

The largest 10% of apples will be sold for export. How large will these apples be?

Trial and error method
Adjust x using the slider until the area to the left is 0.9. The calculations to find the probability (area) from X are exactly those described earlier in this section.

z = (x - mu) / sigma

Direct method
Select this option from the pop-up menu. The calculations are now

This method is better.

(Hint: To fine-tune the slider value, use the arrow keys on the keyboard.)