Carnac The Magnificent
Do you remember this recurring Johnny Carson role where he’d “divine” an answer and then open an envelope to reveal the question? You can do the same with numbers!
Ask your friends to pick a number from 1 to 31. Then double it, add 10, cut that result in half, subtract the number they started with, and—to their astonishment—reveal their result you’ve already written: 5!
It always works! Here’s the math: Let’s say ‘N’ is the number. Double it = 2N. Add 10 = 2N + 10. Cut that result in half = N + 5. Subtract the original N, and you’ve got 5!
I did this once with a class of sixth graders who were visiting me at the office. As we wrapped up, I said I had a “quiz” to give them and offered a prize if their answer (that they’d have to write on their paper) matched a number I had already written down. When we got to the end of the “quiz” and all of us revealed our number 5 at the same time (and they realized they had ALL won a prize), pandemonium broke out!
More than a chant from The Beatles’ “White Album,” the number 9 can help easily spot a transposition error—you know, writing $1.24 instead of $1.42. The difference is 18 cents—a multiple of 9. In fact, the difference between any two transposed numbers will always be a multiple of 9. (Contact me for the algebra behind this!)
This fact could come in handy, for example, when reconciling your checkbook and bank statement. You show a balance of $22,300.54; the bank shows $23,200.54. You’re off by $900. As 900 is a multiple of 9, check your entries for a transposition error.
Golf Card Speed Adding
I play a lot of golf—poorly. Known as a “bogey golfer,” I average a bogey (1 over par) on every hole; so for an 18-hole round, I’ll shoot about a 90. My friends play at pretty much the same level. In fact, the USGA says the average golfer in its national system has a 15 handicap, implying a score of around 90.
Because my golf group members all shoot near 90, I can add our scores very quickly with a little trick called “casting out 5s.” On each nine holes, simply ignore the 5s in your score. You also ignore any pair of scores that add up to 10 (like 7 and 3, 4 and 6). Then add the remaining scores, but don’t use the actual score. Instead, add the difference from 5; so, an actual score of 6 is added as +1; a 4 becomes -1, a 7 (which I see too often on my card…) is a +2. In short, you’ve quickly determined how much your score varies from a score of all 5s. Simply add this variance to 45 to get your front nine score; repeat for the back nine.
Let’s try one. On the front nine, record the following score:
In a split second, I can see that the score is 43. I ignored the 5s on holes 1, 2 and 3. I ignored the 6 and 4 on holes 4 and 5. I ignored the 5 on hole 6. I tallied the 4 on hole 7 as -1; the tally ran to -2 with the 4 on hole 8. I ignored the 5 on hole 9. So, my final tally was -2, which when added to 45 gave me a score of 43.
Robert A. “Rocky” Mills is president of Westlake Investment Advisors in Westlake Village. 805.277.7300. WestlakeIA.com
He is a registered representative with and securities offered through LPL Financial, Member FINRA/SIPC. Investment advice offered through Westlake Investment Advisors, a registered investment advisor and separate entity from LPL Financial.