I assume all these measures are normally distributed, in which case the median and mean are equal.
15a. 
because the normal distribution is symmetric about the mean, and so 50% of families have an income greater than the average.
15b. 
by definition of percentile. The 90th percentile is $178,500, so the top 10% of families earn more than this.
15c. Because it's continuous, this distribution has the property 
. The top 10% earn more than $178,500, so the remaining 90% earn less. 50% of families earn more than the average, so 50% of families earn less than the average. Then 
so 40% of families earn between the median and 90th percentile.
16a. 
or 75% because the first quartile is $570, meaning 25% of renters pay less than this amount.
16b. 
or 50% by the same reasoning used in (15a).
16c. 
or 25% by the same reasoning as in (15c).
