Proability question - 2010 trials (1 Viewer)

[90atarpls]

I dont understand why g1 is 1/2, shouldnt it be 9/21 = 3/7?

so, final answer should be 3/7 x 3/9 = 1/7

 

[zhiying]

I think they select the group first, so irrespective of how many in each group. So you must select group 1 first then select a female out of group 1, and there's 2 groups so 1/2. If he was choosing from a pool of both group 1 and 2, then it asks for whats the probability that a person picked would be group 1, then Pr would be 3/7.

 

[90atarpls]

I think they select the group first, so irrespective of how many in each group. So you must select group 1 first then select a female out of group 1, and there's 2 groups so 1/2. If he was choosing from a pool of both group 1 and 2, then it asks for whats the probability that a person picked would be group 1, then Pr would be 3/7.

Yeah, but the question says she chooses a person, not a group

 

[buriza]

Yeah, but the question says she chooses a person, not a group

yes but first and foremost she has to approach one of the two groups in order to choose a person

therefore you calculate the chance of her approaching the first group which is 1/2 and then selecting a woman out of that group which is 3/9. after that you would multiply the two as the answer states

 

[90atarpls]

but the chance of approaching the groups is not 1/2 as they dont say if the people are spread apart or in groups on different sides of the room, i am just not understanding the question because it doesnt specify.

Tbh, if i was in hsc id write both answers, takes 10 sec extra, would the right one get marked correctly? or would i get 0 marks?

 

[buriza]

but the chance of approaching the groups is not 1/2 as they dont say if the people are spread apart or in groups on different sides of the room, i am just not understanding the question because it doesnt specify.

Tbh, if i was in hsc id write both answers, takes 10 sec extra, would the right one get marked correctly? or would i get 0 marks?

There are two groups of people at a party and Minh is blind-folded. In the first group there are 4 men, 3 women and 2 children. In the second group there are 7 men and 5 women.

Minh is spun around and asked to select one person at random. Find the probability that Minh approaches a person in the first group and then selects a woman.

(1) In these types of questions, assume that each group has an equal chance of being selected unless specified. Do not overthink these questions. They will lead to you getting them incorrect.

(2) Having said that, read the question carefully. Find the probability that Minh approaches a person in the first group and then selects a woman.

(3) The question clearly indicates two steps and most probability questions will be like this, that is, being calculated through sequential steps.

(4) First you think, what is the probability of Minh approaching a person in the first group? 1/2.

(5) Then you think, what is the probability of Minh approaching a woman in the first group? 3/9.

(6) ∴ The answer is 1/2 multiplied 3/9 = 1/6.

And to answer your question, I believe you would not get any marks. You can only write down one answer.

 

[90atarpls]

There are two groups of people at a party and Minh is blind-folded. In the first group there are 4 men, 3 women and 2 children. In the second group there are 7 men and 5 women.

Minh is spun around and asked to select one person at random. Find the probability that Minh approaches a person in the first group and then selects a woman.

(1) In these types of questions, assume that each group has an equal chance of being selected unless specified. Do not overthink these questions. They will lead to you getting them incorrect.

(2) Having said that, read the question carefully. Find the probability that Minh approaches a person in the first group and then selects a woman.

(3) The question clearly indicates two steps and most probability questions will be like this, that is, being calculated through sequential steps.

(4) First you think, what is the probability of Minh approaching a person in the first group? 1/2.

(5) Then you think, what is the probability of Minh approaching a woman in the first group? 3/9.

(6) ∴ The answer is 1/2 multiplied 3/9 = 1/6.

And to answer your question, I believe you would not get any marks. You can only write down one answer.

thanks for the reply/explanation, but im still not understanding why its 1/2, even though there are two groups, there are not the same amount of people in each group

 

[buriza]

thanks for the reply/explanation, but im still not understanding why its 1/2, even though there are two groups, there are not the same amount of people in each group

Look at it this way. There are 2 groups, are there not? The question specifies that Minh is approaching the first group. So what is the chance of Minh approaching one of the two groups (the first group)? Obviously it is 1/2. Do not think about whether there are the same amount of people in each group. At this stage there are 2 groups and that is the only thing you should be thinking about. I believe you may be overthinking the question far more than you need to.

It is as if you are choosing between KFC and McDonalds. There are many meals in KFC and McDonalds, but if you have chosen KFC, you have chosen one out of the two options. Hence 1/2.

 

[90atarpls]

Look at it this way. There are 2 groups, are there not? The question specifies that Minh is approaching the first group. So what is the chance of Minh approaching one of the two groups (the first group)? Obviously it is 1/2. Do not think about whether there are the same amount of people in each group. At this stage there are 2 groups and that is the only thing you should be thinking about. I believe you may be overthinking the question far more than you need to.

It is as if you are choosing between KFC and McDonalds. There are many meals in KFC and McDonalds, but if you have chosen KFC, you have chosen one out of the two options. Hence 1/2.

I thought of that, but thats not the chance of picking a person in the first group... I am srsly not overthinking it lol, but think about this, what if the question was 1 person in group 1 and 99 in group 2, it would not be half, if u walked up to som1 and asked them what group they were in it would be 1/100 chance they were in g1, poorly worded/explained question

 

[90atarpls]

Look at it this way. There are 2 groups, are there not? The question specifies that Minh is approaching the first group. So what is the chance of Minh approaching one of the two groups (the first group)? Obviously it is 1/2. Do not think about whether there are the same amount of people in each group. At this stage there are 2 groups and that is the only thing you should be thinking about. I believe you may be overthinking the question far more than you need to.

It is as if you are choosing between KFC and McDonalds. There are many meals in KFC and McDonalds, but if you have chosen KFC, you have chosen one out of the two options. Hence 1/2.

I think of it more as, 2 groups of marbles, 5 red and 6 blue, probability of picking red is not 1/2, it differs to what they are explaining

 

[buriza]

I thought of that, but thats not the chance of picking a person in the first group... I am srsly not overthinking it lol, but think about this, what if the question was 1 person in group 1 and 99 in group 2, it would not be half, if u walked up to som1 and asked them what group they were in it would be 1/100 chance they were in g1, poorly worded/explained question

But it's not asking the chance of directly picking a person in the first group. It's asking the chance of picking the first group and then picking a person out of that first group. I understand that it is poorly worded but the "then" in the question does indicate it's a two-step answer. Hence the 1/2 x 3/9 = 1/6.

 

[90atarpls]

yes thats what im saying, its not a 50percent chance that youll pick the first group.. i know what you are saying but i just cant believe it because all the crap we do in school is what im saying, and then this question just throws it all away.

Chance of picking the first group is not 1/2, as there are more ppl in the second, sorry for wastin all ur time hah

 

[buriza]

yes thats what im saying, its not a 50percent chance that youll pick the first group.. i know what you are saying but i just cant believe it because all the crap we do in school is what im saying, and then this question just throws it all away.

Chance of picking the first group is not 1/2, as there are more ppl in the second, sorry for wastin all ur time hah

Just try to remember that in questions like this, unless it's otherwise specified, you will have to assume that the chances of picking between the groups is equal. I know it sounds a little bit ridiculous but usually they word these questions poorly and add irrelevant information (in this case the number of people in the second group) to throw people off.

 

[RealiseNothing]

Nah I agree with 90atarplus, I would have put the exact same answer.

Question is dodgy as fuck.

 

[buriza]

Nah I agree with 90atarplus, I would have put the exact same answer.

Question is dodgy as fuck.

lol literally what all the probability questions were like for General Maths (hence why I probably didn't see it as dodgy, since I've gotten used to reading questions like that)

that's probably the only bothersome thing about that subject

 

[RealiseNothing]

lol literally what all the probability questions were like for General Maths (hence why I probably didn't see it as dodgy, since I've gotten used to reading questions like that)

that's probably the only bothersome thing about that subject

The question contradicts itself :/

>Minh spins around and chooses some one randomly (answer is 1/7)
>Minh chooses a group then a woman (answer is 1/6)

Now I understand why English is compulsory.

 

[seanieg89]

How I feel about teachers / courses that assess students with questions like the OP:

 

[seanieg89]

For the record, I agree with 90atarpls and Realise...but questions like these are the reason I would probably get a lower raw mark in general than MX2.

 

[buriza]

For the record, I agree with 90atarpls and Realise...but questions like these are the reason I would probably get a lower raw mark in general than MX2.

It isn't too bad. There's a mix of questions that seem too painfully easy to be true, average questions and then questions like these that you have to read over several times while thinking "why the hell is it worded like this."

 

[seanieg89]

It isn't too bad. There's a mix of questions that seem too painfully easy to be true, average questions and then questions like these that you have to read over several times while thinking "why the hell is it worded like this."

Any ambiguity as to what the question is asking is terrible practice imo. Especially hurting those with high aims.

Such questions really hinder the exams potential as a means for fairly gauging the relative levels of students.