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# S1 (edexcel) question.

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1. I am very good at probabilties questions, but the language of the question confuses me sometimes, e.g.:
1. A group of office workers were questioned for a health magazine and 2/5 were found to take regular exercise. When questioned about their eating habits 2/3 said they always eat breakfast and, of those who always eat breakfast 9/25 also took regular exercise.

2. In a study of 120 pet-owners it was found that 57 owned at least one dog and of these 16 also owned at least one cat. There were 35 people in the group who didn’t own any cats or dogs.

In Q1. 9/25 was P(E I B), whereas in Q2. 16/120 was P(D n C). Both probablities were introduced with the same language, but they meant very different things. How could I know from such language whether it's P(A l B) or P(A n B)???
2. In the first question, they have said that within those 2/3 of people who eat breakfast, 9/25 (of that 2/3) also do regular exercise. You could work out 9/25 of 2/3 and that would be your intersection overall. As P(E l B) is P(E n B) / P(B), which works out to (2/3*9/25)/(2/3), the 2/3 cancels out and you're left with 9/25. It's only because they give it as a probability based on the initial 2/3.
For question two, they're telling you that 57 own a dog, but within these 57 people, 16 of them also own a cat, so they're just telling you the intersection is 16 people out of the total 120.
It's not always as simple as if its a probability it must be conditional, and if numbers it is intersection. Just pay attention to how it's worded and what exactly they are telling you.
3. (Original post by Dominator1)
I am very good at probabilties questions, but the language of the question confuses me sometimes, e.g.:
1. A group of office workers were questioned for a health magazine and 2/5 were found to take regular exercise. When questioned about their eating habits 2/3 said they always eat breakfast and, of those who always eat breakfast 9/25 also took regular exercise.

2. In a study of 120 pet-owners it was found that 57 owned at least one dog and of these 16 also owned at least one cat. There were 35 people in the group who didn’t own any cats or dogs.

In Q1. 9/25 was P(E I B), whereas in Q2. 16/120 was P(D n C). Both probablities were introduced with the same language, but they meant very different things. How could I know from such language whether it's P(A l B) or P(A n B)???
I see what you're saying, but in Q2, P(has a cat given has a dog) would be 16/57, rather than something out of 120, if you see what I mean, whereas for the first one the question implies that the [robability of someone taking regular exercise depends on whether they eat breakfast or not.

I think.. it just takes practice with such questions until you consistently get it right.

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