:4a. Solution.  15 of the families have incomes between 0 and
20,000 dollars a year.  75 have incomes between 20,000 and 50,000
dollars a year.  If we assume that the incomes of these 75
families are distributed evenly over the class interval, it
follows that about a third, or 25, of them have incomes in that
third of the interval from 20,000 dollars to 50,000 dollars a
year.  This results in an estimate of 15 + 25 = 40 families.