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(If D ends respectively in 1, 3, 7, or 9, then multiply by 9, 3, 7, or 1.) Then add 1 and divide by 10, denoting the result as m. Then a number N = 10t + q is divisible by D if and only if mq + t is divisible by D. If the number is too large, you can also break it down into several strings with e digits each, satisfying either 10 e = 1 or 10 e ...
The digit sum of 2946, for example is 2 + 9 + 4 + 6 = 21. Since 21 = 2946 − 325 × 9, the effect of taking the digit sum of 2946 is to "cast out" 325 lots of 9 from it. If the digit 9 is ignored when summing the digits, the effect is to "cast out" one more 9 to give the result 12.
Flow of dollars in the riddle – comparing the sum of values circled in yellow (10+10+10=30) with the sum of absolute values of those shaded yellow (9+9+9+2=29) is meaningless. The missing dollar riddle is a famous riddle that involves an informal fallacy. It dates to at least the 1930s, although similar puzzles are much older. [1]
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The Zig-Zag Girl illusion is a stage illusion akin to the more famous sawing a woman in half illusion. In the Zig-Zag illusion, a magician divides an assistant into thirds, only to have them emerge from the illusion at the end of the performance completely unharmed.
1089 is widely used in magic tricks because it can be "produced" from any two three-digit numbers. This allows it to be used as the basis for a Magician's Choice.For instance, one variation of the book test starts by having the spectator choose any two suitable numbers and then apply some basic maths to produce a single four-digit number.
Digit sums and digital roots can be used for quick divisibility tests: a natural number is divisible by 3 or 9 if and only if its digit sum (or digital root) is divisible by 3 or 9, respectively. For divisibility by 9, this test is called the rule of nines and is the basis of the casting out nines technique for checking calculations.
To remember the order of taxa in biology (Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species, [Variety]): "Dear King Philip Came Over For Good Soup" is often cited as a non-vulgar method for teaching students to memorize the taxonomic classification of system.