Summary: First, fill a 5-liter container with water and pour it into a 3-liter container. At this time, there are 2 liters of water in the 5-liter container. Then pour out the water in the 3-liter container, and then pour the remaining water in the 5-liter container into the 3-liter container. At this time, there are 2 liters of water in the 3-liter container. Finally, fill the 5-liter container with water, and then pour the water into the 3-liter container until it is full. At this point, there are 4 liters of water left in the 5-liter container.
Math:
Mathematics is a subject that studies concepts such as quantity, structure, change, space and information. Mathematics is a universal means for human beings to strictly describe the abstract structure and mode of things, and can be applied to any problem in the real world. All mathematical objects are artificially defined in essence. In this sense, mathematics belongs to formal science, not natural science. Different mathematicians and philosophers have a series of views on the exact scope and definition of mathematics.