maggie27 wrote:
An exam consists of 8 true/false questions. Brian forgets to study, so he must guess blindly on each question. If any score above 70% is a passing grade, what is the probability that Brian passes?
A) 1/16
B) 37/256
C) 1/2
D) 219/256
E) 15/16
Let's solve this using counting methods.
We'll first calculate the TOTAL number of possible outcomes.
This is a true/false test, so each question has 2 possible outcomes.
So the total number of outcomes for all eight questions = (2)(2)(2)(2)(2)(2)(2)(2) =
256Now let's deal with a numerator.
5/8 = 67.5% and 6/8 = 75%
So, in order to get above 70%, Brian must answer 6 or more questions correctly. There are three different cases to consider:
Case i: Brian correctly answers 6 out of 8.
If we let C represent a correct response, and let I represent an incorrect response, then one possible outcome is CCCCCCII, where Brian correctly answers the first 6 questions, and incorrectly answers the last 2 questions
Another possible outcome is IICCCCCC, and ICCCCICC and so on.
So the question boils down to,
"In how many ways can I arrange 6 C's and 2 I's?"---------------------------
When we want to arrange a group of items in which some of the items are identical, we can use something called the MISSISSIPPI rule. It goes like this:
If there are n objects where A of them are alike, another B of them are alike, another C of them are alike, and so on, then the total number of possible arrangements = n!/[(A!)(B!)(C!)....] So, for example, we can calculate the number of arrangements of the letters in MISSISSIPPI as follows:
There are
11 letters in total
There are
4 identical I's
There are
4 identical S's
There are
2 identical P's
So, the total number of possible arrangements =
11!/[(
4!)(
4!)(
2!)]
------------------------
We want to arrange the letters in CCCCCCII
There are
8 letters in total
There are
6 identical C's
There are
2 identical I's
So, the total number of possible arrangements =
8!/[(
6!)(
2!)] =
28So there are
28 different ways to correctly answer 6 questions out of 8
Case ii: Brian correctly answers 7 out of 8.
From here we COULD apply the Mississippi again, but a faster way is to recognize that we just need to place one I in one of the 8 possible locations (first question or second question or third question or... etc)
So there are
8 different ways to correctly answer 7 questions out of 8
Case iii: Brian correctly answers 8 out of 8.
There's only
1 way to correctly answer 8 questions out of 8 (CCCCCCCC)
So the TOTAL number of ways to score better than 70% =
28 +
8 +
1 =
37P(score higher than 70%) =
37/
256Answer: B