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The reason this 'trick' works for all multiples of 11, is because the place value moves over when multiplying more than one place value. This makes it so that the separate place values for the multiple of 11 you are multiplying by is added by the next number in the next place value. This is because 11 has a one in each place, which is why the original multiple doesn't change except for the additional zero at the end that comes when moving to another place value.