Question

A multiple choice test consists of 60 questions. Each question has 4 possible answers of which one is correct. If all answers are random guesses, estimate the probability of getting at least 20% correct. 0.1492 0.3508 0.0901 0.8508 Normal approximation is not suitable.

Answer 1

Answer:

Option 4 - 0.8508

Step-by-step explanation:

Given : A multiple choice test consists of 60 questions. Each question has 4 possible answers of which one is correct. If all answers are random guesses.

To find : Estimate the probability of getting at least 20% correct ?

Solution :

20% correct out of 60,

i.e. $20\%\times 60=\frac{20}{100}\times 60=12$

Minimum of 12 correct out of 60 i.e. x=12

Each question has 4 possible answers of which one is correct.

i.e. probability of answering question correctly is $p=\frac{1}{4}=0.25$

Total question n=60.

Using a binomial distribution,

$P(X\geq 12)=1-P(X\leq 11)$

$P(X\geq 12)=1-[P(X=0)+P(X=1)+P(X=0)+P(X=1)+P(X=2)+P(X=3)+P(X=4)+P(X=5)+P(X=6)+P(X=7)+P(X=8)+P(X=9)+P(X=10)+P(X=11)]$

$P(X\geq 12)=1- [^{60}C_0(0.25)^0(1-0.25)+^{60-0}+^{60}C_1(0.25)^1(1-0.25)^{60-1}+^{60-1}+^{60}C_2(0.25)^2(1-0.25)^{60-2}+^{60}C_3(0.25)^3(1-0.25)^{60-3}+^{60}C_4(0.25)^4(1-0.25)^{60-4}+^{60}C_5(0.25)^5(1-0.25)^{60-5}+^{60}C_6(0.25)^6(1-0.25)^{60-6}+^{60}C_7(0.25)^7(1-0.25)^{60-7}+^{60}C_8(0.25)^8(1-0.25)^{60-8}+^{60}C_9(0.25)^9(1-0.25)^{60-9}+^{60}C_{10}(0.25)^{10}(1-0.25)^{60-10}+^{60}C_{11}(0.25)^{11}(1-0.25)^{60-11}]$

$P(X\geq 12)\approx 0.8508$

Therefore, option 4 is correct.