The mean length of 200 diodes is 20.04mm, with a standard deviation of 0.02mm. What is the probability that a selected diode would have a length less than 20.01mm

Answer 1

20.01 mm is 0.03 mm below the mean ... this is 1.5 standard deviations

use a z-score table to find the portion of the population below -1.5 s.d.

should be around .07

Answer 2

20.01 is 1.5σ below the mean
So look that up in your Z table.