Find P90, the 90-percentile.

Question

Assume that the readings at freezing on a batch of thermometers are normally distributed with a mean of 0°C and a standard deviation of 1.00°C. A single thermometer is randomly selected and tested. This is the temperature reading separating the bottom 90% from the top 10%.

P₉₀ = 

Answer 1

The 90th percentile can be obtained from the standard normal probability table, by looking up the probability closest to 0.9 (0.8997) and finding the associated z-score (1.28). That z-score is the 90th percentile.