Mathemagician Video

[Jim Horn]

Very fun video! I noticed that the fifth two digit number he squared was a multiple of 3, so the square was a multiple of 9 as is the square of that. So the sum of the 7 digit number's digits is also a multiple of 9. By adding the other six called out digits, he could determine the 7th one.

Also, the birthday day of the week calculation can be done with John Horton Conway's remarkable "Doomsday" algorithm. I've used it for years. His calling out the dates by year, month and day is exactly the order used in that algorithm so he could be doing the calculation as the above were called out, yielding the immediate result.

Mental arithmetic - it's fun! Bob Pease once wrote about calculating the common log of 2 in his head to a number of digits. Richard Feynman was known for this skill as well. As someone who celebrated his 25000th day last Friday, I admire those with that skill and try to improve my own.