This is a serious question so please don't add the normal chit chat.

The water in my pool needs refershing. Rather than doing it all at once which can result in problems with the ground under the pool rising, I plan on draining 25% on each of 4 consecutive months. I know that the result will be less than a 100% replacement because the 2nd through 4th times some of the "new" water will be taken out. I'm having difficulty calculating how much "old" water will be left when I'm done. Can you help me figure it out?