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If they are multiples of 9, the digits add up to a multiple of 9.
The tens increase by one while the ones decrease by one. For example 9,18,27, and so on.
Are you asking why? If so, it could be anything. a and b are used because they are easy to relate to.
Used to relate parts or percentages to the whole
they relate because thaey all need to be used to do science