Vote Week1 - Summative Quiz: Question regarding the answer choices

Hi everyone,

Here’s the question:

In a room, there are 200 people.

  • 30 likes only soccer so 30/200 or 0.15
  • 100 likes only basketball so 100/200 or 0.5
  • 70 likes both soccer and basketball so 70/200 or 0.35

What is the probability of a randomly selected person likes basketball given that they like soccer?

The answer choices are

  • 7/20

  • 3/7

  • 7/10

  • 1/2

When I work out this problem, the answer is not one of the choices. I think I have correctly solved the problem using conditional probability. Please advise. Thank you.

The only people who can possibly like basketball and soccer is the 3rd bullet (70 out of 200).
This is known because it is given that the other two populations only like one sport.
Reduce the fraction and that’s 7/20.

Very much appreciate your response TMosh. I am still a tad confused.

The question asks, “What is the probability of a randomly selected person likes basketball given that they like soccer?” From your response, if the answer is 7/20 or 70 out of 200 people which represent people who likes both soccer and basketball, wouldn’t that me P(A intersection B) and not P(A | B) where A is people who likes baseketball and B is people who likes soccer ? The question asks for P(A | B) and not P(A intersection B) if I am not mistaken?

From the Product Rule we have:

(1) P(A intersection B) = P(A) * P(B | A)

and let’s substitute

P(A intersection B) = 70/200 from the problem statement for
people who likes both basketball and soccer.
P(A) = people who likes soccer or 30/200
P(B | A) = people who likes basketball given that they like soccer since A represents people who likes soccer and B represents people who likes basketball.

Therefore, we have, using (1) from above w/substitution:

70/200 = 30/200 * P(B | A) or

70/200 * 200/30 = P(B | A)

70/30 = P(B | A) but 70/30 is not any of the answer choices?

Thank you for any further clarification.

No, I don’t think so. Perhaps you’re reading more into the question than is necessary.

Thank you for your response. If the question is asking for P(A intersection B) then I am really confused why the question is asking " a randomly selected person likes basketball given that they like soccer?" The “given” is specifically stated in the question. Wouldn’t the question be asking what’s the probability of the people liking BOTH soccer and basketball as oppose to liking basketball GIVEN that they like soccer?

I am still very confused unfortunately. If the correct answer is 7/20 then may I suggest to change the wording on the question please.

I think the question just isn’t written very well. The question implies more than it needs to.
I’ll see about asking the staff to update it.

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Note that from the facts given about the statistics, you can say there is no intersection between the two groups that only like one sport. Because, it says they like only one sport. So no intersection is possible.

Thank you for your response. Yes, please ask the staff if they can reword the question.

  • 70 likes both soccer and basketball so 70/200 or 0.35

Thank you. Would that not be an intersection ? The people who likes both soccer AND basketball?

It’s a defined subset of the population. It doesn’t intersect with the other two one-sport-only populations.

Thank you for the prompt response. I am sorry I am not following.

It doesn’t intersect with the other two one-sport-only populations.

But how do we extract that information from the problem statement? The problem statement simply states X # of people likes soccer and Y # of people like basketball and Z # of people likes both soccer and basketball. From that 3 facts alone, how do we extract the information that Z is not an intersectoin of X and Y ? I guess this is where I am still confused that there is no intersection. Thank you for taking the time to respond to my questions. Very much appreciate it.

You’re really thinking about this far more hard than the question requires.

  • You’re given three facts.
  • The question asks you about only one of them, because the other two do not meet the parameters of the question.

Thank you. I understand what you are saying. Please ask the staff to reword the question completely. It is very confusing IMHO. Thank you.

Hi TMosh,

I was traveling and didn’t get a chance to continue our discussion. I tried the solution you suggested and this is the result I got. Please advise. Thank you.

Additionally, the question is about soccer and basketball while the response mentions “football and soccer?” I am confused. Thank you

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I’ll take it up with the course staff. That doesn’t make sense to me.

Agree, thank you TMosh.

Ah, I think I understand it now.

The population being considered in the question isn’t the whole 200 people. The “given that they like soccer” means we’re only considering those who only (30) like soccer, and those who like soccer and basketball (70). So the denominator is 30 + 70 = 100.

The numerator is then 70, because we know the other 30 only like soccer.

So I’m going to go out on a limb and say the correct answer is 7/10.