In the "decimal" system (based on powers of 10), the decimal fraction part gives you fractions over "powers of 10".

For example, if you had the decimal fraction

0.1234

You would increase the denominator by a power of ten for each digit:

0.1234 = 1/10 + 2/100 + 3/1000 + 4/10,000

By putting everything on a common denominator (always the same as the last denominator)

0.1234 = 1000/10,000 + 200/10,000 + 30/10,000 + 4/10,000

0.1234 = 1,234 / 10,000

From there, you can simplify to get the reduced fraction

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Your decimal fraction is

0.2

Using the rule, you get

0.2 = 2/10

This is the fraction given directly by the definition of the decimal system.

Of course, once you have that, you can reduce it to 1/5, or "blow it up" to 2,000/10,000 or 3/15

or any other fraction that has the same value.

0.2 = 2/10

is the same value as

0.20 = 2/10 + 0/100 = 20/100

and so on.

And all these would reduce to 1/5