"Ah, okay. Technically, on the macro scale, that works well enough for simple tasks. Where there are  few variables  and only one or two answers. What we learned at Hogwarts was like... if we rounded gravity from 9.8 up to 10 meters per second to be easier to run the numbers manually and to show your mathematical work. Sure, you can calculate the average length of time it takes to get to Hogwarts on the train if you know the average speed and distance, but you’re ignoring acceleration and deceleration for starting and stopping, taking the sharp turns, and that steep bit of incline near the end. Calculating using the sum of every infinitesimal fraction of time, from the microscopic to the macroscopic. I didn’t lose you there, did I?"