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2. Trachtenberg system - Wikipedia

   en.wikipedia.org/wiki/Trachtenberg_system

   Trachtenberg system. The Trachtenberg system is a system of rapid mental calculation. The system consists of a number of readily memorized operations that allow one to perform arithmetic computations very quickly. It was developed by the Russian engineer Jakow Trachtenberg in order to keep his mind occupied while being in a Nazi concentration camp.

3. Casting out nines - Wikipedia

   en.wikipedia.org/wiki/Casting_out_nines

   To "cast out nines" from a single number, its decimal digits can be simply added together to obtain its so-called digit sum. 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 ...

4. Addition - Wikipedia

   en.wikipedia.org/wiki/Addition

   Addition (usually signified by the plus symbol +) is one of the four basic operations of arithmetic, the other three being subtraction, multiplication and division. [2] The addition of two whole numbers results in the total amount or sum of those values combined. The example in the adjacent image shows two columns of three apples and two apples ...

5. Investigations in Numbers, Data, and Space - Wikipedia

   en.wikipedia.org/wiki/Investigations_in_Numbers...

   In the original edition, there was no multiplication table presented. Instead, students were instructed to color-code multiples of numbers on a 100s chart and evaluate these charts to find common multiples and patterns. There is no formal presentation of decimal addition. Students are instructed to begin by using colored pencils on 10,000 grid ...

6. Digit sum - Wikipedia

   en.wikipedia.org/wiki/Digit_sum

   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.

7. Arithmetic - Wikipedia

   en.wikipedia.org/wiki/Arithmetic

   Arithmetic is the fundamental branch of mathematics that studies numbers and their operations. In particular, it deals with numerical calculations using the arithmetic operations of addition, subtraction, multiplication, and division. [1] In a wider sense, it also includes exponentiation, extraction of roots, and logarithm. [2]