How do you write repeating decimals as fractions?

There's directions in my book,but I don't understand it?
For example, how would you write as a fraction 0.81 with the 81 repeating?

1 answer

multiply the .818181818 by 100.
81.818181818
Now subtract the originalnumber, and get
81
but that is 99 of the original.

81/99 is the equivalent fraction

Now figure out why I multiplied by 100.
What would I have multiplied by if the repeating number were .345345345?