1)  50 ml.
        2)  7
        3)  9

    Notes

  1)  Since average = 2.5, total = 2.5 X 20 = 50 ml.

  2)  If m is the greatest number of walks of over four
      miles, suppose the remaining 20-m are each the minimum
      possible. Then
                       4m + 1.5(20 - m) < 50
			So   m < 8, i.e. m = 7

  3)  If n is the greatest number of walks of over 2.5
      miles, suppose the remaining 17-n are each the minimum
      possible. Then
                       3 X 5 + 2.5n + 1.5(17-n) < 50
                  So   n < 9.5, and n = 9
 

  1). A boy says, "I have as many brothers as sisters". His
      sister says, "I have three times as many brothers as
      sisters". How many children are there in the family?

  2). A family consists of three girls and their parents.
      The average age of the three girls is 15, while the
      average age of all five is 25. The mother is two years  
      younger than the father. How old is the father?

  3). The total of the ages of Archie, Barry and Charlie is
      41 years. When Archie was born, Charlie was half as
      old as Barry is now. When Charlie was as old as Barry
      is now, Archie was 10 years younger than Charlie is
      now. Find the ages of each of the boys.