Perhaps wrote:
After reading all posts above, i got messed up with the basic concepts.
"Let's look at the formula on a simple case. 2 fair coins, and we want to know if we'll get at least one Heads. We'd use the formula to say that the chance of getting heads on the first flip is 0.5, heads on the second is 0.5, and heads on both is 0.5x0.5 or 0.25. This gives us 0.5 + 0.5 - 0.25 = 0.75, which is correct. "
If at least one head is needed in two flips, then propabilty should be " prob of head occuring in first coin (HT--->0.5) + prob of head occuring in second coin (HT--->0.5) + prob of head occuring in both the flips HH--->0.25 ( because we need that at least one head is occurring).
So , all the three probabilties shud be added. Y do v subtract the last one...and how its like " the question being asked here is "at least one head", which means HH is being counted in both calculations of 0.5. That's why we need to remove it afterwards, because it's being double counted, and should only be counted once." ??? ....in ques it is mentioned that "AT LEAST" one head shud occur...so whats wrong in adding the third probabilty in which both heads are occurring.
Hi Perhaps,
I apologize if the example confused you, I'm trying to denote the difference between the standard question:
at least one occurs and this particular question:
Exactly one occursAT LEAST ONE
If at least one occurs, you use the formula P(1) + P(2) - P(1&2). This is because the probability of 1&2 occuring is accounted for once in P(1) and another time in P(2). Hence, it is double counted. You then have to remove one instance of both occuring in order to get the correct answer.
In the simple example of two fair coin flips. The chances of getting Heads on at least one is P(1) + P(2) - P(1&2) or 0.5 + 0.5 - 0.25, which is 0.75. Written out: HH, HT and TH work. TT does not.
EXACTLY ONE
In this question, they are asking the equivalent of getting exactly one Head. Using the same basic formula, we could just take the probability of at least one and subtract the probability of both occuring, which leaves us with P(1) + P(2) - 2xP(1&2). As previously mentioned, the case where both occurs is accounted for twice when it should be accounted for zero times.
For the coin flips we again get P(1) + P(2) - 2xP(1&2) = 0.5 + 0.5 - 2x0.25 = 0.5. Written out: HT and TH work. HH and TT don't work.
On simple examples like this you're probably better off using logic and writing out examples (until ~10 possibilities this is best), but if the exam started asking about 5 coin flips you're better served to use the formulae than writing out 32 possibilities. I hope this helped clear up any misconceptions. The same principle can be applied regardless of the percentages used, it will only change when the number of coinflips or requirements change.
Hope this helps!
-Ron