simple probability (two questions)

I'm not so great with probability, so I used a tutoring website that gives live help that the library pays for. The guy was ok but kicked me off the session due to limited time. He told me that two questions on my sheet were wrong.. Here they are

3. If a letter is picked at random from the set of 26 letters that make up the English alphabet. What is the probability that the letter selected is contained the word "bake" AND the word "math"? I gave 1/26 but he marked that wrong..

4. The Grimaldis have three children born in different years.
A) Draw a tree diagram or list a sample space to show all the possible arrangements of boy and girl children in the Grimaldi family.

flipsy618 said:

I'm not so great with probability, so I used a tutoring website that gives live help that the library pays for. The guy was ok but kicked me off the session due to limited time. He told me that two questions on my sheet were wrong.. Here they are

3. If a letter is picked at random from the set of 26 letters that make up the English alphabet. What is the probability tat the letter selected is contained the word "bake" or the word "math"? I gave 1/26 but he marked that wrong.. How many letters constitute a "successful" selection? In other words, how many cases represent success? What are the total number of cases? Do answering those questions suggest any idea to you about how to proceed.

4. The Grimaldis have three children born in different years.
A) Draw a tree diagram or list a sample space to show all the possible arrangements of boy and girl children in the Grimaldi family. Well, what did you come up with? It is difficult to point out an error in your work if you do not show the work.

Click to expand...

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JeffM said:

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Click to expand...

ok, sorry. i came up with

for 4. (tree diagram)

child 1 ---> b/g
child 2---> b/g
child 3 ---> b/g


which would make a probability of 1/9 but what's wrong with it..?

I made a mistake on number 3 and it was and instead of or..

flipsy618 said:

ok, sorry. i came up with

for 4. (tree diagram)

child 1 ---> b/g
child 2---> b/g
child 3 ---> b/g


which would make a probability of 1/9 but what's wrong with it..?

I made a mistake on number 3 and it was and instead of or..

Click to expand...

The probability of what? According to what you said the problem was, you were to delineate the sample space or construct a tree diagram. Was there a part of the problem that you did not tell us?

What you show above is obviously correct so far as it goes, but it is not a tree diagram

See http://www.usca.edu/Free/mmp_counting/tree_diagrams.html

So a tree diagram would look like this

\(\displaystyle Child\ 1\ \ \ Child\ 2\ \ \ Child\ 3\)

\(\displaystyle \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ B\)

\(\displaystyle \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ B\)

\(\displaystyle \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ G\)

\(\displaystyle \ \ \ \ B\)

\(\displaystyle \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ B\)

\(\displaystyle \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ G\)

\(\displaystyle \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ G\)

\(\displaystyle \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ B\)

\(\displaystyle \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ B\)

\(\displaystyle \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ G\)

\(\displaystyle \ \ \ \ G\)

\(\displaystyle \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ B\)

\(\displaystyle \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ G\)

\(\displaystyle \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ G\)

So the sample space would be {BBB, BBG, BGB, BGG, GBB, GBG, GGB, GGG}, 8 possibilities, not 9.

flipsy618 said:

3. If a letter is picked at random from the set of 26 letters that make up the English alphabet. What is the probability that the letter selected is contained the word "bake" OR the word "math"? I gave 1/26 but he marked that wrong.

Click to expand...

Thank you for correcting the question .. OR is much different from AND!

What is the set of letters that are either in "bake" or in "math"? How many letters are in that set?
The probability of drawing any specific one of those letters is 1/26. Since each letter is mutually exclusive, the probability for the set is the sum of probabilities for the members of the set.

Hmmm. looking back, I can't tell which is really the correct problem. If the word really is "AND" then your answer was correct. But if you told the tutor the word was "OR" he would have marked your answer wrong.

flipsy

Please, please do not respond to a post by editing your original post. It is very easy to miss your edit and assume that you have not responded.