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it is any letter or mathematical symbol that represent a number or a set of number
it is based on the what number the letter represents
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When a symbol is used for a number it could be any symbol at all. It could be a letter from the Roman or Greek alphabet, upper case or lower case. It could also be any random symbol that you like provided that it is not a "reserved" symbol. By that I mean you cannot use "+" as a symbol since that might be very confusing!
Q is the set of all rational numbers. The letter Q is used because rationals can be expressed as a quotient of two integers. Any letter from the Greek or Latin alphabet may be used as a symbol for an individual rational number.