Why the chance of hitting at least one of two 20% chances equals around 36% and not 40%

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Ok so I know most of the reasoning but I just need it explained a bit better because thinking about it is hurtng my brain.

So you have 2 chances of 20% rolls to hit your goal at least once, 2 D5 dice, win condition need one of them to land on 5 would be an example, the chances of either one hitting isn’t 40%, it’s actually more like 36%. I THINK this is because you are hitting 2 out of 2 in some realities so that’s wasted potential, therefore your odds of hitting at least one are lower than 40%… But I really don’t get that last bit at all.

In: Mathematics

10 Answers

Anonymous 0 Comments

20% of the time you’ll hit on the first die, 20% on the other.

However, 4% of the time you’ll hit on both dice, so simply the probabilities together counts that case twice. So you have to remove it, 20% + 20% – 4% = 36%.

Here is another way to look at it. Instead of rolling two dice, you roll on die twice. If you hit on the first roll, you stop, no need to roll again, you already won.

20% of the time you’ll win on the first roll, 80% you roll again. Of that 80 %, you win 20% of the time as well. 80% * 20% = 16%, 20% + 16% = 36%.

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