Here’s a fun and impressive parlor trick with math. I’m sure some of you are familiar with it already (Samuel Beckett mentioned it in his novel Watt, for one thing), but I’m sure some of you aren’t:
Hand someone a calculator. Tell them to choose a number between 1 and 99. Now have them use the calculator to find the cube of that number. Have them tell you the cube, and in a few seconds you can tell them the original number. It’s really impressive, but not very difficult.
Here’s how it works: First, the number you’re working from has to be a perfect cube; that’s important.
Let’s say the cube they give you is 373,248. Look at the last digit and use the following:
(Last digit of cube/last digit of root)
1 / 1
2 / 8
3 / 7
4 / 4
5 / 5
6 / 6
7 / 3
8 / 2
9 / 9
Note that there are no repeats; each digit will result in a uniue corresponging final digit on the cube. Since the last digit of our example is 8, the last digit of the cube root will be 2.
Now strip off the last three digits of the cube, in this case leaving us with 373. You’ll need to memorize the cubes of numbers 1 through 10; not too difficult:
1[sup]3[/sup] = 1
2[sup]3[/sup] = 8
3[sup]3[/sup] = 27
4[sup]3[/sup] = 64
5[sup]3[/sup] = 125
6[sup]3[/sup] = 316
7[sup]3[/sup] = 343
8[sup]3[/sup] = 512
9[sup]3[/sup] = 729
Find the cube that is closest to but LESS than your number. In this case, the cube number is 373, so 343, or 7[sup]3[/sup], is the closest.
So the first digit is 7, second digit 2:
72[sup]3[/sup] = 373,248
Pretty neat trick, I think.