math question

[70kgman]

Lets say event A has a 29.7% chance of happening. and event B has a 42.8% chance of happening. What math is involved in finding out what the chance of EITHER happening is?

Sorry if this is a dumb question. I am drawing a blank, and I have been out of school for a decade. ha.

[Pecos Bill]

add the two

[OverUnder]

72.5 %

[70kgman]

edit

[Pecos Bill]

Are ppl really this dumb?

[Philavanh]

add them

[mathdotcom]

Add the two only if events A and B are independent. Otherwise you also have to subtract the probability of both happening.

http://en.wikipedia.org/wiki/Probability#Mathematical_treatment

[Pecos Bill]

haha dude drew a blank

[forsberg21]

Depends on if the events are independent or not.

[70kgman]

Yes, they are independent.

[70kgman]

Maybe I worded it wrong. Getting heads on at least one of two coin flips is 75%, yes? If I just add 50% + 50% like you told me to do, that is 100% which obviously isn't correct, I am looking for the math involved that gets the 75%.

[forsberg21]

Yes, they are independent.

Ok, so if they're independent, you just add the probabilities together. <--- IGNORE

EDIT: SEE EXPLANATION BELOW, INVOLVES 1 - PROBABILITIES OF NOT HAPPENING MULTIPLIED TOGETHER.

[whatsgood5]

I'll join in with another probably stupid question, but what if both events are over 50% chance???

[Rich Boy]

Maybe I worded it wrong. Getting heads on at least one of two coin flips is 75%, yes? If I just add 50% + 50% like you told me to do, that is 100% which obviously isn't correct, I am looking for the math involved that gets the 75%.

Its quite simple really... You just need to look at this another way...

Getting heads on at least 1 of 2 coin flips is 75%

If you know the odds of getting tails on both coin flips (0.50*0.50=0.25) Thats 25% chance of 2 tails in a row.

So 1-.25 = .75 = 75% chance that 2 tails in a row DOESNT happen

So both heads are 25%, heads then tails is 25% and tails then heads is 25%

25+25+25 = 75%

[Rich Boy]

If you want EXACTLY 1 heads in 2 coin flips, that would be 50%

heads then tails = 25%
tails then heads = 25%

[OverUnder]

I'll join in with another probably stupid question, but what if both events are over 50% chance???

event a 58%
event b 55%

probability = 113/200

[Justin7]

If they are independent, calculate the odds of NEITHER happening.
(1-0.297 ) * (1-0.426 )

The odds of either or both happening are 1 - that number.

[whatsgood5]

event a 58% event b 55% probability = 113/200

Then wouldn't his be 72.5/200?

[Rich Boy]

I'll join in with another probably stupid question, but what if both events are over 50% chance???

If that were the case both events would be dependent on one another so you would have to subtract the overlap before doing the math.

[forsberg21]

I'll join in with another probably stupid question, but what if both events are over 50% chance???

Let's say A = chance of rain on Monday = 70%, B = chance of rain on Tuesday = 70%

You want to find out the chance of EITHER happening. So you take the probability of BOTH NOT HAPPENING minus 1. 30% x 30% = 9%.

1 - 9% = 1 - 0.09 = 91%

So the chances of either event happening is 91%.

For the example in the first post of this thread, you do the same.

[whatsgood5]

My head hurts.

Thanks for trying to clear it up for me though fellas, that last post did help me out a bit.

[70kgman]

Let's say A = chance of rain on Monday = 70%, B = chance of rain on Tuesday = 70%

You want to find out the chance of EITHER happening. So you take the probability of BOTH NOT HAPPENING minus 1. 30% x 30% = 9%.

1 - 9% = 1 - 0.09 = 91%

So the chances of either event happening is 91%.

For the example in the first post of this thread, you do the same.

That is the answer I was looking for. thanks.

[Cheme82]

About 60%