So, when he calculates the time the truck is going to take before crashing, he does it in an instant. Like, 2 small panels are dedicated to it. And that is to give the impression that he did it in an instant. That is not something that you can just do in your head in an instant. Now, it's not a hard calculation. And there are certainly tricks and to remember and calculate instantly, or even having solved something similar earlier could have contributed (they are in high school, so I am assuming basic physics problems would not be off the table)
But in the end of the day, it is something most people could not do in their heads, in an instant that is. (To get an quick estimate I would need at least 10 seconds, and that is in a perfect & ideal case, and it would just be an estimate, and if I was ready for this exact calculation, knowing all the numbers and whatnot, and I am quite quick with numbers. The main character, in an instant, calculated the exact number, in a situation that he just arrived at, and in a situation that probably puts him under pressure)
So, I don't know if this was overlooked by the author or if it is part of the plot. They are clearly both intelligent, but having an understanding of concepts and doing math faster than a calculator are 2 different things. So, I am just hoping that this was not an unintentional mistake by the author and it was actually done with the purpose of establishing just how incredibly smart the protagonist is.
Interesting series though for sure.