Why does watching a video at 1.25 speed decrease the time by 20%? And 1.5 speed decreases it by 33%?

Question

I guess this reveals how fucking dumb I am. I can’t get the math to make sense in my head. If you watch at 1.25 speed, logically (or illogically I guess) I assume that this makes the video 1/4 shorter, but that isn’t correct.

In short, could someone reexplain how fractions and decimals work? Lol

Edit: thank you all, I understand now. You helped me reorient my thinking.

In: 8653

Answer 1

Consider this: It’s impossible to watch a video _infinitely_ fast. You can’t speed it up so much that it takes no time to watch it. But your sense is that increasing the speed by 25% should decrease its length by 25%, and so presumably increasing it 50% should decrease it by 50%, and increasing it by 100% should decrease its length by 100%, which would make it 0 in length, i.e instant, merely by doubling the speed.

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In fact, the speed being a multiplier of the original means it can never end up at 0. If you watched a 5 minute video at 5.0x speed, you can probably intuit that it would take 1 minute to watch – saving you a cool 4 minutes. But if you double that speed again, to 10.0x you can again probably intuit that you won’t save four minutes again. In fact, you’ll only save a further 30s because doubling your speed from 5.0x to 10.0x halves the time to watch it not from the original 5m, but from the already-shortened 1m.

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The moment you speed the video up, it’s no longer “1” in length but rather something smaller than 1. So any additional speed ups will be applying to something less than 1, meaning it’s not a linear relationship. This sounds very complicated, but the maths is incredibly simple:

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1.0 / speed factor = Proportion of the original length

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So to input your own examples, we have…

1.0 / 1.25 = 0.8

1.0 / 1.5 = 0.667

If you multiple those results by 100, you get the percentage change from the original length. And my example:

1.0 / 5.0 = 0.2 (and so if the original video was 5m long, that multiplied by 0.2 = 1m)

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P.S. If anyone ever tells you that they’re going to decrease your pay by 10% but, don’t worry, they’ll then increase that by 20% next month to make up for it, you’re getting screwed for precisely the same reason!