Link Details

This is indeed cool, but has no direct relevance to programming or IT. I'll chip in though... many of these aren't "tricks" so much as a creative manipulation of inputs that helps you to deduce the answer easier (used to use them in math competitions that disallowed writing anything but the answer).

To multiply 2, 2-digit numbers, you can do so like this:

23 * 45 =
1's digit = 3 * 5 = 15 = 5 (remainder 1)
10's digit = (3 * 4) + (2 * 5) + remainder from above = 23 = 3 (remainder 2)
100+'s digit = (2 * 4) + remainder from above = 10

Final answer = 1035

If multiplying a number by 25, you can use (x * 100 / 4), i.e. 16 * 25 = 1600 / 4 = 400. There's other applications here as well (like if multiplying by 50, you can use (x * 100 / 2). You can see that 1600 / 4 is a much easier mental calculation that 16 * 25.

When multiplying 2 numbers, you can apply a factor to one and the inverse to the other, with 2 being the easiest factor, like so:

17 * 44 = 34 * 22 = 68 * 11 = 748

To find the square of a 2-digit number with a 5 for the one's digit, multiply the 10's digit by one higher, and use 25 for the tens digits, like so:

75 squared = (7 * 8) = 56 for the 1000/100's digits
always 25 for the 10/1's digits
= 5625

Cool stuff... I guess doing math in your head is soo old school... unfortunately, as people have no clue as to whether the results they are getting are reasonable or not. Cheers for all your tips! I'm thinking about incorporating them into my post ;)

Feel free, I just wish I could remember all of them!

And yeah, most people don't do many mental calculations anymore. It was actually pretty fun while in college, most of the classes that "required" a calculator for tests and quizzes really didn't need one at all in most cases. Usually they were multiple-choice, in which case the answers were off by a factor of 10 or more, so a quick mental calculation was more than accurate.

LOL I should almost report you for cheating at the exams! :-P

Very enjoyable!