Is 1.25 a rational number and why?

A rational number is one that can be written in the form of one whole number over another whole number.

1.25 = 125/100 which is one whole number over another whole number.

Thus 1.25 is a rational number.

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If p and q are integers with q ≠ 0, then p/q is a rational number.

The decimal expansion of a rational number either terminates or ends with a repeating sequence of one or more digits (this latter occurring when there is a prime factor other than 2 and/or 5 in the denominator).