Is there a formula for adding percentages together?

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I am doing a study and want to add several percentages together to get an odd. But whenever I add the percentages together, it never comes out right. I want the number to be under 100 percent, but it always comes out over. I've tried researching different ways to do it, but it keeps coming out wrong. Does anyone know of any way?

I can probably help. What exactly are you trying to do? Can you supply the numbers? Also what is an 'odd'.

Can you explain the percentages in more detail? Such as what they are portions of?

Okay thank you very much! By odd I mean the odds, like 1 out of 5 type odds for example. But maybe I can worry about the odds later. So far the percentages I want to add up 85%, 91%, and 76%. There maybe more to add, but that's what I have so far. But cannot add it and come out under 100.

Sorry, what do you mean by 'portions of', when you ask?

^ what are those percentages of? An example would be 75% of the apples are red, and 20% are green, and 5% are yellow.

Dont know what exactly you mean by "adding percentages".

You would have to give examples.

Do you mean like: ten percent of the sample plus 20 percent of the same sample? Ten percent of drivers in Ohio drive red cars, and 20 percent of drivers in Ohio drive blue cars? In that example you can add the two together and say that 30 percent of Ohio drivers drive cars that are either red or blue.

Or do you mean ten percent of sample A, and 20 percent of sample B? Ten percent of Ohio drivers drive red cars and 20 percent of Indianna drivers also drive red cars? In that case you cannot add the percentages because they are percentages of different totals. If you want to state what the "percentage of Indianna and Ohio drivers who drive cars that are red" is...then you would have to know the actual numbers of drivers in both states, add those numbers together, and then add the actual number of red cars in both states together. And only then calculate what the percentage of red cars is out of the combined numbers.

If you're talking about "odds" then it sounds like you need multiply rather than add.

The odds of a dice coming up six is one sixth, or about 17 percent. And the odds of it coming up odd is 50 percent. So if you throw two dice the odds that one will come up six and the other comes up as any odd number would be 17 percent of 50 percent. Or 8.5 percent. I think.

Of each of those percentages is out of a hundred on different statistics, if that is what you mean?

Can you provide the specific details?

Like Natural Plastic said, you can't add percentages from different samples.

Perhaps you're saying you want to know the odds of pulling a red jelly bean from one bag and a green m&m from a separate bag?

Yes the chance of all three occuring simultaneously, yes. If there is a way to figure that out?

0.85 * 0.91 * 0.76 = 0.59% of all three happening at same time if each probability is independent of the other.

Oh okay. So I was doing a study on birth control and it turns out that using condoms has a 85% success rate. Using the pill has a 91% success rate. And having unprotected sex without ejaculation has a 76% percent rate.

So if a couple of having sex were to do all three, how much would the percent of success be then? And if possible, could you explain how to do that formula too ?

Well ...as I understand what you're saying...you know the odds of A happening, and the odds of B happening, and the odds of C happening. So you want to know what the odds are of A and B,and C, ALL happening at once?

In that case you would convert the percentages into decimals, and...not add...but multiply. Resulting smaller, rather than larger odds.

So if your postman has a ten percent chance of getting bitten by a dog on his route, and the odds that the dog is a pitbull is ten percent, and if 50 percent of all dog breeds are female, then what are the odds that your postman will be bitten by a female pitbull on a given day? It would be point one times point one, times point five. Or half of one tenth of one tenth. Which would be one/200, or "one half of one percent".

Source
https://www.statisticshowto.com/how-to-find-the-probability-of-two-events-occurring-together/

The formula doesn't work for this example because using a condom and having unprotected sex cannot both occur at the same time. Also its the probability of all three working simultaneously if it did work. The pill is still going to work 91% of the time.

Oh okay. Is there an entirely different formula that I would use for that then, to figure out the percentage of success if all three were used?

I didn't mean unprotected sex with a condom, sorry. What I mean is, using a condom without ejaculating, and using the pill as well. Is there a way to figure out all three in a percentage of success rate?