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  2. Mental calculation - Wikipedia

    en.wikipedia.org/wiki/Mental_calculation

    Since 9 = 10 − 1, to multiply a number by nine, multiply it by 10 and then subtract the original number from the result. For example, 9 × 27 = 270 − 27 = 243. This method can be adjusted to multiply by eight instead of nine, by doubling the number being subtracted; 8 × 27 = 270 − (2×27) = 270 − 54 = 216.

  3. Trachtenberg system - Wikipedia

    en.wikipedia.org/wiki/Trachtenberg_system

    Some of the algorithms Trachtenberg developed are ones for general multiplication, division and addition. Also, the Trachtenberg system includes some specialised methods for multiplying small numbers between 5 and 13. The section on addition demonstrates an effective method of checking calculations that can also be applied to multiplication.

  4. Mental abacus - Wikipedia

    en.wikipedia.org/wiki/Mental_abacus

    Mental calculation is said to improve mental capability, increases speed of response, memory power, and concentration power. Many veteran and prolific abacus users in China, Japan, South Korea, and others who use the abacus daily, naturally tend to not use the abacus any more, but perform calculations by visualizing the abacus.

  5. Mental calculator - Wikipedia

    en.wikipedia.org/wiki/Mental_calculator

    A mental calculator or human calculator is a person with a prodigious ability in some area of mental calculation (such as adding, subtracting, multiplying or dividing large numbers). In 2005, a group of researchers led by Michael W. O'Boyle, an American psychologist previously working in Australia and now at Texas Tech University , has used MRI ...

  6. Chisanbop - Wikipedia

    en.wikipedia.org/wiki/Chisanbop

    With the chisanbop method it is possible to represent all numbers from 0 to 99 with the hands, rather than the usual 0 to 10, and to perform the addition, subtraction, multiplication and division of numbers. [4] The system has been described as being easier to use than a physical abacus for students with visual impairments. [5]

  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]

  8. Zerah Colburn (mental calculator) - Wikipedia

    en.wikipedia.org/wiki/Zerah_Colburn_(mental...

    Zerah was reported to have been able to solve fairly complex problems. For example, the sixth Fermat number is 2 2 5 +1 (or 2 32 +1). The question is whether this number, 4,294,967,297, is prime or not. Zerah calculated in his head that it was not and had a divisor of 641. (Its other proper divisor is 6,700,417).

  9. Multiplication - Wikipedia

    en.wikipedia.org/wiki/Multiplication

    Multiplication can also be thought of as scaling. Here, 2 is being multiplied by 3 using scaling, giving 6 as a result. Animation for the multiplication 2 × 3 = 6 4 × 5 = 20. The large rectangle is made up of 20 squares, each 1 unit by 1 unit. Area of a cloth 4.5m × 2.5m = 11.25m 2; 4 ⁠ 1 / 2 ⁠ × 2 ⁠ 1 / 2 ⁠ = 11 ⁠ 1 / 4 ⁠

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