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11 0 (Take the last digit of the number, and check if it is 0 or 5) 11 0 (If it is 0, take the remaining digits, discarding the last) 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)
The positive integers may be partially ordered by divisibility: if a divides b (that is, if b is an integer multiple of a) write a ≤ b (or equivalently, b ≥ a). (Note that the usual magnitude-based definition of ≤ is not used here.) Under this ordering, the positive integers become a lattice, with meet given by the gcd and join given by ...
Two properties of 1001 are the basis of a divisibility test for 7, 11 and 13. The method is along the same lines as the divisibility rule for 11 using the property 10 ≡ -1 (mod 11). The two properties of 1001 are 1001 = 7 × 11 × 13 in prime factors 10 3 ≡ -1 (mod 1001) The method simultaneously tests for divisibility by any of the factors ...
The following laws can be verified using the properties of divisibility. They are a special case of rules in modular arithmetic, and are commonly used to check if an equality is likely to be correct by testing the parity of each side. As with ordinary arithmetic, multiplication and addition are commutative and associative in modulo 2 arithmetic ...
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.
11: sum of digits in even places and sum of digits in odd places differ by a multiple of 11. 6: a number that passes the divisibility tests for both 2 and 3. 7: sum of all blocks of three digits is a multiple of 7. 9: successive subtraction of final three digits from all the other digits yields a multiple of 9. Base 15 (odd but not prime):
Demonstration with Cuisenaire rods of the amicability of the pair of numbers (220,284), the first of the series.. In mathematics, the amicable numbers are two different natural numbers related in such a way that the sum of the proper divisors of each is equal to the other number.
Rule of nines (mathematics), a test for divisibility by 9 involving summing the decimal digits of a number Wallace rule of nines , used to determine the percentage of total body surface area affected when assessing burn injuries