Probability Question quick Q (1 Viewer)

[karnage]

In a class of 30 students, 22 study chem, 18 study physics and 13 study both. If one student is chosen at random, what is the probability that she/he studys chem OR physics

So without adhering to any formula, i used a venn diagram + logic

So the answer i get is (9/30) + (5/30) = 14/30

But the answer excel gets is 9/10
They used the non mutually exclusive formula i.e. (22/30) + (18/30) - (13/30) = (9/10)

Why? Please explain.

 

[Timothy.Siu]

well, i guess the questions ambiguous should have sed chemistry and/or physics

 

[aakash]

well, i guess the questions ambiguous should have sed chemistry and/or physics

OR actually means and/or

 

[karnage]

Hmmmm....

so is that what non mutually exclusive means ?

and/or ?

Edit:

From my study notes

mutually exclusive: cannot both occur at the same time
non mutually exclusive: can have some occur at the same time (i guess this is where the and/or ties in?)
product rule for independent events: they can both occur at the same time, result of 1 stage isnt affected by previous stages

Is this right?

Thanks guys, much appreciated

 

[Forbidden.]

回复: Probability Question quick Q

In a class of 30 students, 22 study chem, 18 study physics and 13 study both. If one student is chosen at random, what is the probability that she/he studys chem OR physics

So without adhering to any formula, i used a venn diagram + logic

So the answer i get is (9/30) + (5/30) = 14/30

But the answer excel gets is 9/10
They used the non mutually exclusive formula i.e. (22/30) + (18/30) - (13/30) = (9/10)

Why? Please explain.

P(A or B) = P(A) + P(B) - P(A and B)

If you only did (22/30) + (18/30) you get (40/30) and you will overcount because the sum of probabilities cannot be greater than 1 !!!! (Remember this !!!!)

Hence (22/30) + (18/30) - (13/30) = (9/10)

By using (9/30) + (5/30) = 14/30 you are counting Physics indivdually AND Chemistry individually but didn't include the "BOTH" Physics and Chemistry to account for all who study Physics and Chemistry.

 

[aakash]

Hmmmm....

so is that what non mutually exclusive means ?

and/or ?

no.

mutually exclusive means that the 2 events cant occur together
example-
=>getting a 5 on a dice
=>getting a 6 on a dice
since u cant have them both simultaneously

since studying chem and studying physics can occur simultaneously hence they are NOT MUTUALY EXCLUSIVE. Also Mut. exc. events dont have anything in common (ie the 'circles' dont cross each other in venn diagram)

 

[karnage]

no.

mutually exclusive means that the 2 events cant occur together

I said non mutually exclusive

So, whenever it says OR, it can be either and both?
i.e. non mutually exclusive can be either and both as well?

Thats acceptable i suppose (i have trouble accepting things until i learn it rofl)

Thanks for the quick replies

 

[aakash]

i.e. non mutually exclusive can be either and both as well?

???
being mutually exc or not is NOT related to or/and.

yepp i know u said non-mutually exclusive- but the best to understand non-mutually exclusive evets is to understand mutually exclusive- coz either 2 events can be ME or NME...

dont be confused, simply use

if they are mutually exclusive then you use P(A or B) = P(A) + P(B)
if they are non-mutually exclusive then use P(A or B) = P(A) + P(B) - P(A and B)

yepp A or B means A or B or both

 

[Timothy.Siu]

that just confuses me, just do the question lol.

 

[karnage]

???
being mutually exc or not is NOT related to or/and.

yepp i know u said non-mutually exclusive- but the best to understand non-mutually exclusive evets is to understand mutually exclusive- coz either 2 events can be ME or NME...

dont be confused, simply use

if they are mutually exclusive then you use P(A or B) = P(A) + P(B)
if they are non-mutually exclusive then use P(A or B) = P(A) + P(B) - P(A and B)

yepp A or B means A or B or both

Ok its all good

Thanks once again.

Btw thats one kickarse GPA, actuarial studies as well :jaw:

 

[Timothy.Siu]

Ok its all good

Thanks once again.

Btw thats one kickarse GPA, actuarial studies as well :jaw:

thats wat i was thinking. no offence but compared to ur uni marks u did pretty crap for 3&4unit

 

[aakash]

Ok its all good

Thanks once again.

Btw thats one kickarse GPA, actuarial studies as well :jaw:

LOL tnx, but there are a few with 4 gpa damn them lol coz i just missed out on gpa 4 with 4 marks in one unit lol

anywayyy goodluck with ur hsc

 

[aakash]

thats wat i was thinking. no offence but compared to ur uni marks u did pretty crap for 3&4unit

yeh thats what i think, maybe trying for a selective could have helped lol coz in my school i was coming 1st easilyy in 4 unit so there was no competetion, i wanst even expecting to get in actuarial, but who cares about hsc now, all it matters is getting a high gpa so i get a job lol.

 

[Timothy.Siu]

yeh thats what i think, maybe trying for a selective could have helped lol coz in my school i was coming 1st easilyy in 4 unit so there was no competetion, i wanst even expecting to get in actuarial, but who cares about hsc now, all it matters is getting a high gpa so i get a job lol.

yeah true, after the hsc is over and u get in the course it means nothing