Two methods: 1) Fill measuring cup B with water and pour it into measuring cup A. At this time, measuring cup A contains 300 ml of water ... Fill cup B with water and cup A with water. At this time, there is 500 ml of water in cup a,

There is 100ml water left in cup B. Pour out the water in Cup A, and pour the water in Cup B into Cup A. At this time, Cup A has 100ml of water, then fill Cup B with 300ml of water and pour it into Cup A..

There is 400 ml of water in a cup.

2) Fill Cup A with water and Cup B with water. At this time, there are 300ml in Cup B and 200ml in Cup A. Pour out the water in Cup B, and then pour the water in Cup A into Cup B. ..

At this time, Cup A is empty, and Cup B still has 200ml of water. Fill cup A with water, and fill cup B with water from cup A.. At this time, there is 300ml of water in cup B, leaving cup A.

400 ml water.