I'm guessing you don't spend 100% of your time in front of a computer, and even if you do, in the time it takes you to do that, someone who can actually do mental arithmetic will have approximately calculated that problem and several more, because they can stay in the "flow" of thinking about the bigger problem. As someone who can, it's astounding how many developers out there can't --- pulling out the calculator for computations as simple as 12+51 or 6x128.
It reminds me of the "developers don't need to learn how to type quickly" argument --- yes, you can be productive without, but you'll quickly find yourself to be in a handicap in contrast to others who can write, rewrite, and mentally estimate and compute the results in their head several times faster, saving much time in compiling/running/debugging/etc.
(I'm fairly sure there's a famous book about this kind of thing but for some reason my brain is stuck on "How To Solve It" by Polya which I don't think is correct.)
Here is the 2017 final of the 'proper, serious' Countdown, also on YT https://www.youtube.com/watch?v=Q-VdiufVWrw
I seem to remember one where the hostess wrote the numbers on a huge dick she'd drawn on the board. Thus giving new meaning to "Numberwang".
I know HOW the calculation works and why, but I don't need to know and remember times tables. I like the idea of thinking outside the box on how to teach math and science, as I feel modern schools were built with factories and the industrial complex in mind, not technology, arts, and sciences.