4. A sociology professor finds that his student's scores on an exam are normally distributed with population mean = 80 and population standard deviation = 6. Find the 40th percentile.

5. Use the following data for problems 5.a and 5.b. Assume the height of 10 year old boys in the US is normally distributed. Also assume that the population mean is = 54 inches and the population standard deviation is = 1.6 inches. 5.a What proportion of 10-year-old boys are between 51 inches to 57 inches tall? 5.b Find the 25th percentile.

6. Use the following data for problems 6.a and 6.b. The incubation time (time to hatch) for chicken eggs is normally distributed with population mean = 21 days with a population standard deviation = 1 day. 6.a What proportion of eggs require more than 22.4 days to hatch? 6.b What proportion of eggs hatch between 18.5 to 23.5 days?