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I got a knot in my brain and can't think of this - I'm sure it's quite simple and I'll be like "duh!" once someone points it out to me...

So the problem is as follows:

An audio file is 30 seconds long. That's 100% of the audio file.

If I play the file at 50% of it's speed it'll be 60 seconds long.

At what percentage do I need to play the song to make it X seconds long? (I am looking for 46.56)

Or in other words if 30 is to 100 the same as 60 is to 50, then what would 46.56 be equivalent to?

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To make it $X$ seconds long, play it at $\frac{30}{X}\times 100$ per cent speed.

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Since the 50 and 100 refer to "speed" whereas the 30 and 60 refer to "length", it's actually "$100$ is to $1/30$ (i.e., play $1/30$ of the file per second) as $50$ is to $1/60$ (play $1/60$ of the file per second)".

That is, $$\frac{\mbox{speed factor}}{1/\mbox{desired length}} = \frac{{100}}{1/{30}}.$$

Equivalently, $$(\mbox{speed factor}) (\mbox{desired length}) = 100 \cdot 30 = 3000.$$

$$\mbox{speed factor} = \frac{3000}{\mbox{desired length}}.$$

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