Help! quick maths help needed

Greebo

Greebo

Greebo

Caporegime
Joined
20 Jan 2005
Posts
45,777
Location
Co Durham
My brain is failing me today and I quickly need the answer to the following.

In the next 18 months there are 7 months of importance. I know for sure that 4 other important things will occur in the next 18 months.

What is the probability that one of the 4 things coincide with one of the 7 months of importance.

Needs a quick answer, help please. Greatly appreciated.
 
Any one month has a 7/18 chance of being a month of importance.

Any one month has a 4/18 chance of an important thing occurring.

Since any one of the seven months can occur in tandem with any one of the 4 important things, I reckon the answer is to add them to get 11/18. Please wait for others though...
 
Robbie G said:
Any one month has a 7/18 chance of being a month of importance.

Any one month has a 4/18 chance of an important thing occurring.

Since any one of the seven months can occur in tandem with any one of the 4 important things, I reckon the answer is to add them to get 11/18. Please wait for others though...
Click to expand...

You need to multiply I believe, not add. hence, 7/18 x 4/18 is your probability.
 
Chance of a certain time in the next 18 months being an important month:

7/18

Now you have 4 chances of this happening so ... well I know you don't just add them together but I can't remember what you do next.
 
kgi said:
You need to multiply I believe, not add. hence, 7/18 x 4/18 is your probability.
Click to expand...

Pretty sure that's not true, in that case you get a really low probability whereas the answer will definitely be higher then 7/18.

7/18 is the likelihood of it happening if there is only 1 special event.
 
kgi said:
You need to multiply I believe, not add. hence, 7/18 x 4/18 is your probability.
Click to expand...

No, I don't think that's right. That's what it would be if you needed all 4 of the things to fall in one of seven months, I believe...it comes out at less than 10%, which doesn't appear high enough for this.
 
Ye gads.

It's easier to work out the probability of it not occurring - which is (11/18) to the power 4 - and subtracting that from 1.

So your probability of one or more conflicts is 86%
 
Just to clarify as I may not have explained it properly.

There will be 7 important months in the next 18. This is a 100% certainty. The only thing not known is which 7.

There will also be 4 other different important events cropping up in the next 18 months. This is also 100% certainty.

Therefore I am trying to find out what the chance is that any one of the 7 important months coincides with one of the 4 important events.

It does not matter if more than one occurs eg all 4 coincide or just 3 or 2. Just need to know if at least one does.

Gut feeling is that multiplying is wrong as the answer is too low.....
 
Mr Jack said:
Ye gads.

It's easier to work out the probability of it not occurring - which is (11/18) to the power 4 - and subtracting that from 1.

So your probability of one or more conflicts is 86%
Click to expand...

That's looking favourite so far and makes sense. ANy more confimations that this is the right answer please?
 
Oh, also, does it matter if any of the 4 other important things coincide with each other?

That would make it 1-(11/18)(10/18)(9/18)(8/18) = 92%
 
Let's look at a similar problem. Suppose you have two coins and you flip them, what's the probability that they will both be heads?

I think it's the same type of problem. You got 1/2 chance of the first coin coming out heads (i.e. 7/18 months) and again 1/2 chance of the second one (i.e. 4/18). To find the answer you need to multiply. I stand by my answer which comes at around 8.6%.
 
Mr Jack said:
Oh, also, does it matter if any of the 4 other important things coincide with each other?

That would make it 1-(11/18)(10/18)(9/18)(8/18) = 92%
Click to expand...

Yes it does. Never thought of that. None of the 4 events can occur in the same month.
 
Multiplication Rule
The multiplication rule is a result used to determine the probability that two events, A and B, both occur.

The multiplication rule follows from the definition of conditional probability.


The result is often written as follows, using set notation:

where:
P(A) = probability that event A occurs
P(B) = probability that event B occurs
= probability that event A and event B occur
P(A | B) = the conditional probability that event A occurs given that event B has occurred already
P(B | A) = the conditional probability that event B occurs given that event A has occurred already


For independent events, that is events which have no influence on one another, the rule simplifies to:

That is, the probability of the joint events A and B is equal to the product of the individual probabilities for the two events.
 
kgi said:
Let's look at a similar problem. Suppose you have two coins and you flip them, what's the probability that they will both be heads?

I think it's the same type of problem. You got 1/2 chance of the first coin coming out heads (i.e. 7/18 months) and again 1/2 chance of the second one (i.e. 4/18). To find the answer you need to multiply. I stand by my answer which comes at around 8.6%.
Click to expand...

No. It's more like the old problem of what are the odds if you have 30 people in a room together that two people have exactly the same birthday. Most people work it out to be really low. The actual answer is near enough 50% which surprises most people.

(this assumes that every day is just as likely to be somebodies birthday which in real life isn't true is AUgust/September has the highest birthrates - xmas and new year "accidents" plus major power cuts cause peaks as well ;))
 
kgi said:
Let's look at a similar problem. Suppose you have two coins and you flip them, what's the probability that they will both be heads?

I think it's the same type of problem. You got 1/2 chance of the first coin coming out heads (i.e. 7/18 months) and again 1/2 chance of the second one (i.e. 4/18). To find the answer you need to multiply. I stand by my answer which comes at around 8.6%.
Click to expand...

How about we pick an example which isn't entirely misleading? And actually bares some resemblance to the OP's problem. Try this one: you're going to toss two coins and see whether either of them come up tails. This is much closer to the original question because it's asking for one or more, rather than all of.

Multiplying the chances is absolute mathematical garbage akin to trying to work out 3+6 by appending the digits to get 36.
 
kgi said:
That is, the probability of the joint events A and B is equal to the product of the individual probabilities for the two events.
Click to expand...

Yes, that's entirely correct. It's absolutely nothing to do with the problem you've been asked to solve though.

(edit) What you've actually worked out is the a priori probability that both will fall in any given month without knowing what happened in all the other months; which is clearly different from what is asked.
 
Last edited:
Isn’t it more like: The chances of having a bomb on a plane is a million to one? And the chances of having two bombs on a plane are a million times a million to one? And if that’s correct you are safer taking your own bomb with you?

Just a thought!
 
Mr Jack said:
Ye gads.

It's easier to work out the probability of it not occurring - which is (11/18) to the power 4 - and subtracting that from 1.

So your probability of one or more conflicts is 86%
Click to expand...

Thinking about it because none of the 4 events can occur in the same month, shouldn't it be

1- (11/18 * 10/17 * 9/16 * 8/15) ?

as once you have the probability that the first event didn't coincide you only have 10 months of the 17 months left for the 2nd event to occur in.

Or am I over analysing this?
 
You must log in or register to reply here.
Back
Top Bottom