A bottle containing 10 liters of alcohol one liter of alcohol is removed and replaced with water. After a liter of the mixture was removed and replaced with water, the operation being carried out 7 times in total. Then that amount of alcohol remains in the container.
after each step, there is 0.9 as much alcohol as before.
So, after 7 steps, the amount of alcohol is 10*0.9^7 = 4.78 liters.