5-2 = 3 bottles

750-450 = 300g

300/3 =100g

So the weight of water is100g.

Three bottles of water and an empty bottle.

450g 450-3× 100 = 150g。

So the bottle is150g.

Primary school mathematics, including Olympiad Mathematics, needs appropriate methods in learning. With good methods and ideas, you may get twice the result with half the effort!

Thinking in images means that people use thinking in images to understand and solve problems. Its thinking foundation is concrete image, and its thinking process is developed from concrete image. The main means of thinking in images are objects, figures, tables and typical image materials.

Its cognitive feature is that it is average in individual performance and always retains its intuition about things. Its thinking process is represented by representation, analogy, association and imagination.

Its thinking quality is manifested in actively imagining intuitive materials, processing and refining appearances, and then suggesting the essence, laws, or looking for objects. Its thinking goal is to solve practical problems and improve thinking ability in solving problems.