Rational numbers are denoted by Q because we basically know that every rational numbers are quotient. So we clearly understand that every rational numbers are given in a fraction method where denominator will never be zero and numerator will be given as a whole number.
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Rational number are denoted as Q. The word  from which it is derived is 'quoziente', which is a italian word, meaning quotient since every rational number can be expressed as a quotient or fraction p/q of two co-prime numbers p and q, q≠0. It was first denoted by Peano in 1895.