Category Archives: Math

Math tricks

Today’s New York Times has the obit of arithmetic wizard Shakuntala Devi:

 In 1977, at Southern Methodist University in Dallas, she extracted the 23rd root of a 201-digit number in 50 seconds, beating a Univac computer, which took 62 seconds.

In 1980, she correctly multiplied two 13-digit numbers in only 28 seconds at the Imperial College in London. The feat, which earned her a place in the 1982 edition of the Guinness Book of World Records, was even more remarkable because it included the time to recite the 26-digit solution.

(The numbers, selected at random by a computer, were 7,686,369,774,870 and 2,465,099,745,779. The answer was 18,947,668,177,995,426,462,773,730.)

For the record, it took me 20 seconds to read the answer aloud, counting the time I needed to determine the number is in the septillions.

Her mental legerdemain was sharpened by the fact that her father was a lion tamer, a fact thatI hope gives you incentive to read the entire Times obit.

To show off for Times journalists in 1977, she answered the following:


Answers below the fold.

Meanwhile, she wrote some books. For one of them, Amazon’s sneak peak shows, among other things, how to determine if a number is divisible by 7. I’ve been looking for that for years. Her method is a bit of brute force, but it works.

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Math problems

This is from a study guide my fourth-grade daughter received:


The calculation error gives the teacher the right answer, instead of 1/24.

Normally, I give stuff like this a pass, since anyone can make a mistake. But this particular teacher is a math specialist, and it’s the second time a study guide has had both an error in arithmetic and algorithm design.


Pi games

Yesterday (3/14) was pi day, but I only saw this link today, to a game that estimates pi by dropping sticks on a cyber-mat.

Try it! And be sure to scroll up to read the explanation why it works.


Is your birthday special?

Only quants can have fun with dates that doesn’t involve snogging.

Financial expert/blogger David Markel defines a special birthday as one where you can create a simple math equation from the date.

For example, David was born Dec. 5, 1960. That, of course, is 12/5/60, and 12 x 5 = 60. There are also special days for:

  • Addition (1/1/02 is an example)
  • Subtraction – (12/5/07)
  • Division – (8/1/08)

Fun idea, right? Not sure how you count European dating, where the date precedes the month, i.e., 12/5/07 would be 5/12/07.

Brushing that aside, here’s the big question: Over a 400-year arc (has to be 400 to properly handle the leap-year cycle), how many birthdays are special?

The answer, and the methodology, are here.