Is - 6.175 is an irrational number?

No.

Any number you can write completely is a rational number.

Any number with a repeating decimal fraction is also a rational number.

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Any number that goes on forever without repeating is an irrational number. These are usually represented symbolically (because they cannot be written "exactly" any other way). These include such numbers as √2, π, e, ∛ (-4), and an infinite number of others.