lets say there is 100ml per cup
and 10 ml per spoon
u took out 10ml of the milk into the coffee which leaves 90ml of milk in the cup and 10ml of milk in the 110ml of mixture of milk and coffee
at this time, the content of coffee+milk is 100ml of coffee and 10ml of milk
which makes the percentage of the milk = 9.01%
and then you take a spoonful of coffee+milk mixture into the milk, you are actually taking 100%-9.01% coffee in the 10ml which is approx 8.99ml of coffee