There are 5 spoons of tea in one glass and 5 spoons of milk in another. One spoon of milk was poured from the second glass into the first one, and then one spoon of tea with milk was poured from the first glass into the second. Which concentration is bigger: tea in milk or milk in tea?
The amount of poured off milk from the second glass is exactly the same as the amount of poured tea. Thus, the concentrations of milk in tea and tea in milk are equal.