Math Is Fun Forum

NESIC

Member

Registered: 2010-09-25

Posts: 16

Probability

Dear all kindly guide me towards solution for Part B

Professor Random has taught probability for many years. She has found that 80% of students who do homework pass the exam, while 10% of students who don’t do the homework pass the exam. If 60% of the students do the homework,
A.what percent of students pass the exam?
B.Of students who pass the exam, what percent did the homework?

Ans:
Part:A
P(Pass who do HW)=.8
P(Fail Who do HW)=.2
P(Pass how not do HW)=.1
P(Fail Who not do HW)=.9
P(Pass if 60% do HW)=52%

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Probability

Hi;

I agree.

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,583

Re: Probability

Hi Nesic

The diagram below should help you reach a solution.

I started with 100 students (no loss of generality as you want percentages).

You can then work out the values 48 and 12 using the information given.

There are 40 students who don't do HW so you can work out 4 and 36.

Part A can be found by adding values to get the number in Pass.

And part B by calculating the percentage in the overlap as a percent of those in Pass.

Hope that helps.

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

Murray

Guest

Re: Probability

I don't think the last two posts realised you needed help. However, you know that 52% of people passed the exam. Still from the overall percentage 48% did homework and passed, so therefore the percentage of people that passed and did homework is 48/52 which is about 92%.
Hope that helps, I had to keep questioning it while I was typing

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Probability

Hi Murray;

I don't think the last two posts realised you needed help.

Perhaps that is justified with the first poster. He does not like just answering questions without spurring the questioner on to doing a little bit on their own.

But it is not justified for bob bundy's reply which answers both A and B by pointing out how to correctly reason about this problem.

---

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.