The weights of male babies less than 2 months old in the United States is normally distributed with a mean of 11.5 pounds and a standard deviation of 2.7 pounds.

How do I find the 80th percentile for these weights?

2 answers

Look up in your Z table for the upper tail at 20%

You can play around with this at

http://davidmlane.com/hyperstat/z_table.html

where you can set the mean and variance and explore various properties of the normal distribution
You can divide it by the 80th percentile.