This is something very low-level. It has to do with the IEEE representation of a float/double. One thing you might try is multiplying each number by one hundred, using integer, perform your calculation, and dividing back by one hundred. You might also try a different rounding mechanism such as POSIX::ceil or floor. Only use the number of digits of precision you need, "convert" that into an integer, perform the addition, and "convert" back.

What I don't understand is your title. If you start with floats, then you'll end with floats.

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