I'm searching for a formula which can calculate the remaining video speed if selected different speeds.
So if I have a video and watch it twice as fast, then it's half the time. That's easy. But what should I calculate if I use 1.25, 1.5 or 1.75 times speed?
And how can I use the formula the opposite so when watching with slower speed like 0.25 or 0.5?
$\frac{T_1}{T_2} = \frac{S_2}{S_1}$, where $T_1, T_2$ refer to times (lengths or durations of time) and $S_1, S_2$ refer to speeds of the videos.
Note the reversed ratios, because the speed and time length are inversely proportional.
Example of application: you have a video length 5 min at normal (1x) speed. What is the length (in time) at 1.75x speed?
$\frac{T_1}{T_2} = \frac{S_2}{S_1}$
$\frac{5}{T_2} = \frac{1.75}{1}$
So $T_2 = 2.857...$ minutes, which is approximately $2$ minutes $51$ seconds.
For slower than normal speeds (like 0.5x), just put that in place of $S_2$.