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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