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  2. Sum-product number - Wikipedia

    en.wikipedia.org/wiki/Sum-product_number

    The natural numbers 0 and 1 are trivial sum-product numbers for all , and all other sum-product numbers are nontrivial sum-product numbers. For example, the number 144 in base 10 is a sum-product number, because 1 + 4 + 4 = 9 {\displaystyle 1+4+4=9} , 1 × 4 × 4 = 16 {\displaystyle 1\times 4\times 4=16} , and 9 × 16 = 144 {\displaystyle 9 ...

  3. Mental calculation - Wikipedia

    en.wikipedia.org/wiki/Mental_calculation

    The product of the 2 one-digit numbers will be the last two digits of one's final product. Next, subtract one of the two variables from 100. Then subtract the difference from the other variable. That difference will be the first two digits of the final product, and the resulting 4 digit number will be the final product. Example:

  4. Persistence of a number - Wikipedia

    en.wikipedia.org/wiki/Persistence_of_a_number

    The search for these numbers can be sped up by using additional properties of the decimal digits of these record-breaking numbers. These digits must be in increasing order (with the exception of the second number, 10), and – except for the first two digits – all digits must be 7, 8, or 9.

  5. Digital root - Wikipedia

    en.wikipedia.org/wiki/Digital_root

    The digital root (also repeated digital sum) of a natural number in a given radix is the (single digit) value obtained by an iterative process of summing digits, on each iteration using the result from the previous iteration to compute a digit sum. The process continues until a single-digit number is reached.

  6. Digit sum - Wikipedia

    en.wikipedia.org/wiki/Digit_sum

    In mathematics, the digit sum of a natural number in a given number base is the sum of all its digits. For example, the digit sum of the decimal number 9045 {\displaystyle 9045} would be 9 + 0 + 4 + 5 = 18. {\displaystyle 9+0+4+5=18.}

  7. Arithmetic - Wikipedia

    en.wikipedia.org/wiki/Arithmetic

    For example, in the method addition with carries, the two numbers are written one above the other. Starting from the rightmost digit, each pair of digits is added together. The rightmost digit of the sum is written below them. If the sum is a two-digit number then the leftmost digit, called the "carry", is added to the next pair of digits to ...

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    mail.aol.com

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  9. Napier's bones - Wikipedia

    en.wikipedia.org/wiki/Napier's_bones

    The product's digits will stand in the order as calculated left to right. Apart from the first and the final digit, the product's digits will each be the sum of two values taken from two different bones. Third step of solving 6 x 425. Bone values are added together, as described above, to find the digits of the product.