First, write it out. 6.55 = 6

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We need to simplify. Let's start by dividing the numerator and denominator by compatible numbers. I'll use 5 to start.
55÷5 = 11. 100÷5 = 20.
You cannot simplify

further, as 11 is a prime number.
Therefore, 6.55 in a mixed number form is 6

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Hope that helped you.