My math teacher put this question on a quiz today and I am 100%

>>304534
P(A OR B) = P(A) + P(B) - P(A AND B)

>>304536
Doesn't that only work if the two events can occur at the same time?

>>304543
they can occur at the same time, that's the 3rd probability. 25% chance of falling asleep and passing

>>304543
Also if they can't occur at the same time P(A AND B) just equals 0

>>304549
Gee whiz I feel like an idiot.

>>304534
Maybe I'm dumb but isn't it 75%?

>>304556
Pretty sure that's the chance of falling asleep and failing the exam.

>>304549
Wait a minute that doesn't add up.
"Falling asleep AND passing the exam" implies that this is happening in sequence.

>>304556
It's the probability of doing one or the other but not both. So you take the probability of one happening plus the probability of the other happening, minus the probability of both happening.
40+70-25=85
>>304560
You're right, I wasn't very clear. Time doesn't really matter here. They either happen, or they don't. So you can fall asleep in class and pass. What I should have said was probability of both events occuring

40% plus 70% minus the 25% overlap is 85%.

>>304557
This is wrong. I assume you got this by doing 100% and taking away the chance of falling asleep and passing the exam. But you're missing the options of staying awake and passing/failing.

>>304557

Retard.

File: maths4.jpg (63KB, 539x960px) Image search: [Google]

63KB, 539x960px

>>304557
Here, it's easier if you do a venn diagram. Everything inside the box has to equal 100%. Everything in sleep circle has to equal 40%, and everything in pass circle has to equal 70%. And there is the overlap of 25%. As you can see, the chance of falling asleep and failing, which i have coloured green for you, is 15%

>>304594
Wow, that helps way more than the way my teacher did it.
I'm gonna use that method from now on.

>>304595
I'm surprised your teacher didn't use it. Really makes everything so much easier. Happy mathsing