Work out 5 first. The height of 5L water after pouring into the container, and then the height of reducing water with 15 is the height of water discharged by the apple volume, and then the volume of the apple can be calculated.

Or: After the apples are put in, the height of water and apples is 15. Count the water and apples, and then subtract the water.

When you encounter problems, you should think for yourself. When you can't figure it out, drawing is easy to understand.