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](https://tex.z-dn.net/?f=20%5C%25%5Ctimes%2060%3D%5Cfrac%7B20%7D%7B100%7D%5Ctimes%2060%3D12)
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](https://tex.z-dn.net/?f=p%3D%5Cfrac%7B1%7D%7B4%7D%3D0.25)
Total question n=60.
Using a binomial distribution,
![P(X\geq 12)=1-P(X\leq 11)](https://tex.z-dn.net/?f=P%28X%5Cgeq%2012%29%3D1-P%28X%5Cleq%2011%29)
![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)]](https://tex.z-dn.net/?f=P%28X%5Cgeq%2012%29%3D1-%5BP%28X%3D0%29%2BP%28X%3D1%29%2BP%28X%3D0%29%2BP%28X%3D1%29%2BP%28X%3D2%29%2BP%28X%3D3%29%2BP%28X%3D4%29%2BP%28X%3D5%29%2BP%28X%3D6%29%2BP%28X%3D7%29%2BP%28X%3D8%29%2BP%28X%3D9%29%2BP%28X%3D10%29%2BP%28X%3D11%29%5D)
![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}]](https://tex.z-dn.net/?f=P%28X%5Cgeq%2012%29%3D1-%20%5B%5E%7B60%7DC_0%280.25%29%5E0%281-0.25%29%2B%5E%7B60-0%7D%2B%5E%7B60%7DC_1%280.25%29%5E1%281-0.25%29%5E%7B60-1%7D%2B%5E%7B60-1%7D%2B%5E%7B60%7DC_2%280.25%29%5E2%281-0.25%29%5E%7B60-2%7D%2B%5E%7B60%7DC_3%280.25%29%5E3%281-0.25%29%5E%7B60-3%7D%2B%5E%7B60%7DC_4%280.25%29%5E4%281-0.25%29%5E%7B60-4%7D%2B%5E%7B60%7DC_5%280.25%29%5E5%281-0.25%29%5E%7B60-5%7D%2B%5E%7B60%7DC_6%280.25%29%5E6%281-0.25%29%5E%7B60-6%7D%2B%5E%7B60%7DC_7%280.25%29%5E7%281-0.25%29%5E%7B60-7%7D%2B%5E%7B60%7DC_8%280.25%29%5E8%281-0.25%29%5E%7B60-8%7D%2B%5E%7B60%7DC_9%280.25%29%5E9%281-0.25%29%5E%7B60-9%7D%2B%5E%7B60%7DC_%7B10%7D%280.25%29%5E%7B10%7D%281-0.25%29%5E%7B60-10%7D%2B%5E%7B60%7DC_%7B11%7D%280.25%29%5E%7B11%7D%281-0.25%29%5E%7B60-11%7D%5D)
![P(X\geq 12)\approx 0.8508](https://tex.z-dn.net/?f=P%28X%5Cgeq%2012%29%5Capprox%200.8508%20)
Therefore, option 4 is correct.