so, in real world application, what's the most useful/valuable topic in mathematics?
Although I wonder for 90% people of this world that math is just a tool to pass the exam at school, not any real application (or they just can't sense it).
It depends on what your relative judgements are for what is deemed 'valuable' or 'useful'. Are the number theory foundations that allow digital transactions to occur more valuable than the subtraction done to figure out how many minutes until the next hour?
Although I wonder for 90% people of this world that math is just a tool to pass the exam at school, not any real application (or they just can't sense it).