Pour 300ml into the cup first, then pour it out, then pour 300ml into the cup, then pour 300ml into the 500ml cup, then pour 300ml into the 300ml cup, and then pour the 500ml cup into the 500ml cup. At this time, there is 100ml of water left in the 300ml cup. Pour out the water in the 500ml cup, and pour the remaining 65438+ into the pot.

In this way, there is still 100 ml of water left in the two cups.