To be honest, I just know 36 times 24. I've spent enough time dealing with numbers with all their prime factors small that I can just see it's 864. It might als

To be honest, I just know 36 times 24. I've spent enough time dealing with numbers with all their prime factors small that I can just see it's 864. It might also help that I know there are 24 hours in a day, 3600 seconds in an hour, and 86,400 seconds in a day -- and I probably have a lot of random facts like this somewhere in my head that I at least occasionally pin mental arithmetic problems on.