When is the last time this happened to you? You are stranded on a deserted
island without a calculator and for some reason you must determine if 67 is
a divisor of 95733553; furthermore, a coconut recently fell on your head and
you have completely forgotten how to perform long division.
Of course the above scenario would never happen (we all carry around
calculators) but it's good to know that if we should ¯nd ourselves in a similar
situation there is an easy divisibility rule for 67: remove the two rightmost
digits from the number (in our case, 53), double them (106) and subtract
that from the remaining digits (957335¡106 = 957229); the original number
is divisible by 67 if and only if the resulting number is divisible by 67. If the
resulting number is not obviously divisible by 67 we can repeat the process
until we get a number that clearly is or is not a multiple of 67. In the above
example, we get the following.
This article has two aims. First, to identify six categories of tests that
most divisibility tricks fall into, and second, to provide an easy divisibility
test for each number from 2 { 102 (thus the 101 Ways..." in the title). We'll
see that in fact many numbers have more than one divisibility test.
When is the last time this happened to you? You are stranded on a desertedisland without a calculator and for some reason you must determine if 67 isa divisor of 95733553; furthermore, a coconut recently fell on your head andyou have completely forgotten how to perform long division.Of course the above scenario would never happen (we all carry aroundcalculators) but it's good to know that if we should ¯nd ourselves in a similarsituation there is an easy divisibility rule for 67: remove the two rightmostdigits from the number (in our case, 53), double them (106) and subtractthat from the remaining digits (957335¡106 = 957229); the original numberis divisible by 67 if and only if the resulting number is divisible by 67. If theresulting number is not obviously divisible by 67 we can repeat the processuntil we get a number that clearly is or is not a multiple of 67. In the aboveexample, we get the following.This article has two aims. First, to identify six categories of tests thatmost divisibility tricks fall into, and second, to provide an easy divisibilitytest for each number from 2 { 102 (thus the 101 Ways..." in the title). We'llsee that in fact many numbers have more than one divisibility test.
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