As someone with moderate Japanese literacy, I don't really see the distinction between the problems that you're referencing. All problems here are using the sam

As someone with moderate Japanese literacy, I don't really see the distinction between the problems that you're referencing. All problems here are using the same mix of phonetic and logographic characters, which is typical of modern written Japanese. >Did Japan import whatever concept question 2 is about from China and developed the math for the other questions independently? Very unlikely. Japanese concepts expressed by the borrowed Chinese logographic script (known locally as kanji) are frequently quite different than the meaning that would be expressed in Chinese. Additionally, choosing to write a text with relatively dense usage of kanji characters doesn't necessarily imply anything meaningful. For one, the swap between phonetic script and kanji often plays the role that the empty 'space' character plays in western languages. Sometimes the concepts being discussed include words that are homophones/homographs that are difficult to distinguish between in phonetic script, but are easy to distinguish with kanji. For instance, 'hashi' = chopsticks or bridge depending on emphasis. Aside from use of context it's not necessarily clear what you mean if you spell it out, but there are two distinct kanji for either 'chopsticks' or 'bridge' that are usually known, so it's easier to use those to ward off ambiguity. In other cases, choices between use of phonetic scripts or kanji simply comes down to nothing more than convention.