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Willis Nelson Dysart (15 March 1923, in Omega, Georgia – 8 November 2011) [1] was an American mental calculator. His talent for arithmetic emerged at the age of three after his mother taught him to count. He quit school in the third grade (age 9) and pursued a career as a lightning calculator. [2] [3] [4] [5]
11 × 2 = 22 (Multiply the result by 2) 110 ÷ 5 = 22 (The result is the same as the original number divided by 5) If the last digit is 5. 85 (The original number) 8 5 (Take the last digit of the number, and check if it is 0 or 5) 8 5 (If it is 5, take the remaining digits, discarding the last) 8 × 2 = 16 (Multiply the result by 2)
The products of small numbers may be calculated by using the squares of integers; for example, to calculate 13 × 17, one can remark 15 is the mean of the two factors, and think of it as (15 − 2) × (15 + 2), i.e. 15 2 − 2 2.
Soon after their development by Genaille, the rulers were adapted to a set of rods that can perform division. The division rods are aligned similarly to the multiplication rods, with the index rod on the left denoting the divisor, and the following rods spelling out the digits of the dividend. After these, a special "remainder" rod is placed on ...
So for example, for a promptuary capable of multiplying two five-digit numbers together, the strips should 6 times as long as they are wide, with 50 number strips and 50 mask strips. Napier's example specified strips 1 finger (19mm) wide and 11 fingers (209mm) long, enabling the device to multiply two 10-digits numbers to produce a 20-digit result.
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
Following the example above, say you put 20 percent down on a home costing $400,000, and you get a 30-year fixed-rate mortgage to finance the remaining $320,000 at 6.6 percent interest.
This is because there are integers that 7 may be multiplied by to reach the values of 14, 49, 0 and −21, while there are no such integers for 3 and −6. Each of the products listed below, and in particular, the products for 3 and −6, is the only way that the relevant number can be written as a product of 7 and another real number: