What is 3 and three fifths as a fraction?

Expressed as a mixed number, 3 3/5 is already expressed in its simplest form.

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Another way, you can also express it as 18/5, but notice, if the question asks you to express in its simplest form, then 3*3/5 is the correct answer, other wise, you can just leave it as 18/5 if you want.