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  • A multiple choice exam has 175 questions.
  • Each question has 4 possible answers.
  • Only 1 answer out of the 4 possible answers is correct.
  • The pass rate for the exam is 70% (123 questions must be answered correctly).
  • We know for a fact that 100 questions were answered correctly.

Questions: What is the probability of passing the exam, if one were to guess on the remaining 75 questions? That is, pick at random one of the 4 answers for each of the 75 questions.

Kendra
  • 61

3 Answers3

5

We have already answered 100 questions, so there are only 75 questions left to answer. Since we are guessing our way through the multiple choice questions, our probability of success in each question will be $\frac{1}{4}$ Since the pass mark is $\frac{123}{175}$, we need at least $\frac{23}{75}$ in the final 75 questions. This is the same as saying that we need to find $P(X\ge23)$, i.e. "What is the probability of getting 23 or more questions correct?"

The information we have so far suggests that we can use the binomial distribution. $X \sim B(n,p)$. Where $n=75$ and $p=\frac{1}{4}$ in your question. However, we may have a slight problem. 75 is too large for us to use the $ncr$ formula and binomial tables don't generally include $n=75$. Unless you have a graphical calculator or some sort of statistical software, we will need to use a $normal \ approximation$ in order to answer your question.

When do you need to normally approximate?

  • Look at $np$ and $nq$. (For your question, n=75 and p=$\frac{1}{4}$)
  • Look at n, is it "Large"? ($n\ge30$ is normally a candidate).
  • if $n$p and $nq>5$ and $n$ is large, you can try a normal approximation.
  • Also, if $p$ is close to $\frac{1}{2}$, this is another indication.

$X \sim B \left(75,\frac{1}{4} \right)$

we are looking for $P(X\ge23)$

Normally approximating:

$Y \sim N(18.75,14.0625)$ as $Y \sim N(np,npq)$ is the normal approximation to $X \sim B(n,p)$ where $q=1-p$

Applying continuity correction, $P(X \le 22)$ becomes $P(Y \le 22.5)$

Normal distribution: $Z = \frac{X-\mu}{\sigma}$

$Z= \frac{22.5-18.75}{\sqrt14.0625}$

We need to find $1-P(Z<1)$

Look for $\varPhi(1)$ on normal tables, the answer is $0.84134$.

Therefore, the normal approximation gives us an answer of $1-0.84134=0.15866$

The normally approximated answer to your question is $0.159(3dp)$.

Continuity correction:

$P(X\ge23)$ is the same as $1-P(X\le22)$

$P(X\le a)$ becomes $P(Y\le a+0.5)$ after the continuity correction.

user4568
  • 503
1

Hint : Use the formula for binomial distributed random variables.

Peter
  • 84,454
1

The question is essentially what the probability of answering more than 22 questions out of 75 correctly.

The chance that we will answer all 75 incorrectly is simply $(3/4)^{75}$. The probability that we will answer one particular question correctly and all the rest incorrectly is $(1/4)(3/4)^{74}$. Thus, the probability that we will answer exactly one question incorrectly is $75(1/4)(3/4)^{74}$ since there are 75 possible choices to answer correctly.

In general, the probability that $p$ questions will be answered correctly will be $${75 \choose p}\left(\frac{1}{4}\right)^p\left(\frac{3}{4}\right)^{75-p}$$ and thus our total probability is

$$P = \sum_{p=23}^{75} {75 \choose p}\left(\frac{1}{4}\right)^p\left(\frac{3}{4}\right)^{75-p} \approx 0.158$$

Joshua Mundinger
  • 4,678
  • It's faster to do $1 - P(fail)$ or $1 - \sum_{p = 0}^{22} {75 \choose p}.25^p.75^{75-p}$, which computes to the exact same value, of course. – Avraham Nov 04 '14 at 20:39