I'll give this one a try !!!!
The volumes in the glasses don't really matter since they're the same......and the amount we put into each glass doesn't matter, either.......as long as it's the same...
Suppose there are 9 teaspoons of each liquid in each glass.
Taking a tsp of water and putiing it into the glass of milk results a mixture of milk and water that is 9 parts milk and 1 part water. {We assume this is well-mixed !!! } Also, there are now 8 tsp of water left in the pure water glass.
Now taking a tsp of the water/milk mixture and putting it into the pure water glass results in a mixture of 8.1 tsp water and .9 tsp milk....a 9 : 1 ratio
But....the original pure milk glass now contains 9 tsp of the water/milk mixture....and 9 parts of this is milk (8.1 tsp) and 1 part is water (.9 tsp)
So......each glass is equally contaminated......the original pure milk glass contains .9 tsp of water and 8.1 tsp of milk.....a 1 : 9 ratio
And the original pure water glass contains .9 tsp of milk and 8.1 tsp of water....again, a 1 : 9 ratio.
