64-16=48L of milk. When topped up with water the amount of milk is now 48/64=3/4 (75%). This leaves 1/4 (25%) water.
64-16=48L of solution. 3/4 of this solution was milk and 1/4 water. So the amount of milk drawn off this time is 3/4 of 16=12L. So, in all 16+12=28L of milk have been drawn off, leaving 64-28=36L. The concentration of milk is now 36/64=9/16. Water must therefore be 7/16 of the solution (28L).
64-16=48L of solution. 9/16 of the 16L=9L milk. In all, 16+12+9=37L have been drawn off, leaving 64-37=27L of milk, a concentration of 27/64. Water content is now 37/64 (57.8125%) or 37L.