# Homework Help: The probability that a new drug will cure a skin rash is 0.82.

1. Jun 21, 2011

### Mspike6

1. The problem statement, all variables and given/known data

"The probability that a new drug will cure a skin rash is 0.82. if the drug is administered to 200 patients with the skin rash, find the probability that:

a)more than 150 will be cured?
b)between 170 and 180, inclusive will be cured?
"

2. Relevant equations

3. The attempt at a solution

i really dont know how to solve it, i missed the class for this one and i have no clue. I think i will have to use the z-score table, but it doesnt make sense, and i never had to get a probability of a probability.

any help or hint will be greatly appreciated, Thanks !

p.s. I didnt know where to post my question, so if am posting in the wrong section, i apologize

Last edited: Jun 21, 2011
2. Jun 21, 2011

### AJKing

Re: Statistics!!Probqbility

I believe you're in the right section.

What do you know about combinations and permutations?
Also, have you have you learnt much about the Binomial Theorem?

Last edited: Jun 21, 2011
3. Jun 21, 2011

### Mspike6

Re: Statistics!!Probqbility

Yes i learnd about the binomial theorem, but how can i apply in this question

4. Jun 21, 2011

### AJKing

Re: Statistics!!Probqbility

Well, there's a derivative of the Binomial Theorem that can be applied to situations where you want to find out the probability of an event that has only two out comes.

For instance, flipping a coin results in a head or a tail.

Guessing the answer to a question on a test results in a success or a failure.

Look at the equation below and I'll describe some parts.

$P (N of X) = _{X} C _{N} (b)^N(q)^{X-N}$

$X =$ the total number of items in a given system. (50 questions on a test, 32 flips of a coin, Etc.)

$N =$ the number of successful out comes you want to evaluate. (exactly 12 correct questions, exactly 28 heads)

$b =$ the probability of a successful outcome occurring. (1/4 if there are 4 options on a test, 1/2 if there are 2 sides to a coin)

$q =$ the probability of an unsuccessful outcome. (3/4 chances to fail each question on the test, 1/2 chances to flip a tail)

Can you see how your question can apply to this?

This is relative to the binomial theorem in a way that I wouldn't be too great at describing with out looking at some notes. But, here's some logical intuition.

If you're going to flip a coin 4 times, and I ask you "What are the odds of you getting 3 heads and 1 tail?" You might multiply the probability of each event together, like so:

$1/2 * 1/2 * 1/2 * 1/2 = 1/16$

This would be the same as:

$(1/2)^3 * (1/2) = 1/16$

However, this only applies for 1 instance. To find out how many times this event occurs, we would use Combinations like so:

$_{4} C _{3} = 4$

Using these pieces of information together will tell us the total probability of flipping 3 heads and 1 tail, and bring us back to the Binomial Theorem.

$P ( 3 heads of 4 flips ) = _{4} C _{3}(1/2)^3(1/2) = 1/4$