To tell you what it means in any specific context, you'd have to supply us with that context.
However, speaking generally...
"Back forty" usually is a reference to an piece of land, specifically forty acres of land. "Back" could be geographical (the "back" part of the land, i.e. that furthest away from some reference point like a house or road) or metaphorical (the land that hasn't been developed yet, as opposed to land that has been made suitable for farming).
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A back forty is a remote part of a farm, or, idiomatically, the most remote or inaccessible part of somewhere.
Forty is a natural number that comes after thirty-nine and before forty-one. It is represented by the numeral 40.
If you mean sixty-eight billion and forty-five thousandths, then it'd be 68,000,000,000.045. If you mean sixty-eight billion forty-five thousands, then it'd be 68,000,045,000.
4,011 <3 idiot
40.545 in word form is 'Forty point five four five'. NB NOT 'forty point five hundred and forty five'.