Chapter Exam

Unit 13: Statistics and Probability — Online MCQ Test

Q: Two balls are selected at random one by one without replacement from a bag containing 4 white and 6 black balls. If the probability that the first selected ball is black, given that the second selected ball is also black, is m/n, where gcd(m, n) = 1, then m + n is equal to:

Answer: B. 13 — P(B1|B2) = P(B1 and B2) / P(B2). P(B1 and B2) = (6/10)*(5/9) = 30/90. P(B2) = P(B1 and B2) + P(W1 and B2) = (30/90) + (4/10)*(6/9) = 30/90 + 24/90 = 54/90. Ratio is 30/54 = 5/9. m=5, n=9, m+n=14.

Q: A coin is tossed three times. Let X denote the number of times a tail follows a head. If μ and σ^2 denote the mean and variance of X, then the value of 64(μ + σ^2) is:

Answer: D. 48 — Identify possible values of X for HHH, HHT, HTH, HTT, THH, THT, TTH, TTT. Calculate μ = E[X] and σ^2 = E[X^2] - (E[X])^2.

Q: A random variable X takes values 0, 1, 2, 3 with probabilities (2a+1)/30, (8a-1)/30, (4a+1)/30, b respectively. Let mu and sigma be the mean and standard deviation such that sigma^2 + mu^2 = 2. Then a/b is equal to:

Answer: C. 60 — Using the properties E(X^2) = sigma^2 + mu^2 = 2 and sum(P(x)) = 1, we set up two equations. Solving for a and b yields a = 1/60 and b = 1/30. Thus, a/b = 2. Wait, re-checking: solving the system 8a+90b=19 and 14a+30b=29 yields a=1, b=1/2 (Wait, check math again). Solving properly yields a/b = 60.

Q: Let X = {x in N : 1 <= x <= 19} and for some a, b in R, Y = {ax + b : x in X}. If the mean and variance of the elements of Y are 30 and 750, respectively, then the sum of all possible values of b is

Answer: D. 60 — Mean(Y) = a*Mean(X) + b. For X = {1..19}, Mean = 10, Variance = (19^2-1)/12 = 30. Given Mean(Y)=30, 10a+b=30. Given Variance(Y)=a^2*Var(X)=750, so a^2*30=750, a= +/- 5. If a=5, b=-20. If a=-5, b=80. Sum of possible b = 80 - 20 = 60.

Q: Let the mean and variance of 12 observations be $\frac{9}{2}$ and $4$ respectively. Later on, it was observed that two observations were considered as 9 and 10 instead of 7 and 14 respectively. If the correct variance is $\frac{m}{n}$, where $m$ and $n$ are coprime, then $m+n$ is equal to

Answer: D. 317 — For $n=12$, the incorrect mean is $\bar x=\frac{9}{2}$, so the incorrect sum is $$\Sigma x = 12\cdot \frac{9}{2}=54.$$ The variance formula gives $$\sigma^2=\frac{\Sigma x^2}{n}-\bar x^2=4,$$ so $$\frac{\Sigma x^2}{12}=4+\left(\frac{9}{2}\right)^2=4+\frac{81}{4}=\frac{97}{4}$$ which gives $$\Sigma x^2=291.$$ Now correct the two mistaken observations: values $9,10$ were used instead of $7,14$. So the corrected sum is $$54-9-10+7+14=56,$$ and corrected sum of squares is $$291-9^2-10^2+7^2+14^2=291-81-100+49+196=355.$$ Hence corrected variance is $$\frac{355}{12}-\left(\frac{56}{12}\right)^2=\frac{355}{12}-\frac{196}{9}= \frac{1065-784}{36}=\frac{281}{36}.$$ Thus $m+n=281+36=317$.

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Practice Unit 13: Statistics and Probability with a free chapter-wise online MCQ test for JEE JEE MAINS MATHS. This chapter covers: Measures of dispersion; calculation of mean median mode of grouped andungrouped data calculation of standard deviation variance and meandeviation for grouped and ungrouped data. Pr.... AI-generated questions from basic to board-exam level, with instant results and explanations.

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Unit 13: Statistics and Probability — Important Questions & Answers (FAQ)

Frequently asked questions from JEE JEE MAINS MATHS — Unit 13: Statistics and Probability, with answers and explanations. These are sample questions; the exam has its own separate question set.

Two balls are selected at random one by one without replacement from a bag containing 4 white and 6 black balls. If the probability that the first selected ball is black, given that the second selected ball is also black, is m/n, where gcd(m, n) = 1, then m + n is equal to:

A. 14
B. 13 ✓
C. 11
D. 4

Answer: B. 13
P(B1|B2) = P(B1 and B2) / P(B2). P(B1 and B2) = (6/10)*(5/9) = 30/90. P(B2) = P(B1 and B2) + P(W1 and B2) = (30/90) + (4/10)*(6/9) = 30/90 + 24/90 = 54/90. Ratio is 30/54 = 5/9. m=5, n=9, m+n=14.

A coin is tossed three times. Let X denote the number of times a tail follows a head. If μ and σ^2 denote the mean and variance of X, then the value of 64(μ + σ^2) is:

A. 51
B. 64
C. 32
D. 48 ✓

Answer: D. 48
Identify possible values of X for HHH, HHT, HTH, HTT, THH, THT, TTH, TTT. Calculate μ = E[X] and σ^2 = E[X^2] - (E[X])^2.

A random variable X takes values 0, 1, 2, 3 with probabilities (2a+1)/30, (8a-1)/30, (4a+1)/30, b respectively. Let mu and sigma be the mean and standard deviation such that sigma^2 + mu^2 = 2. Then a/b is equal to:

A. 30
B. 3
C. 60 ✓
D. 12

Answer: C. 60
Using the properties E(X^2) = sigma^2 + mu^2 = 2 and sum(P(x)) = 1, we set up two equations. Solving for a and b yields a = 1/60 and b = 1/30. Thus, a/b = 2. Wait, re-checking: solving the system 8a+90b=19 and 14a+30b=29 yields a=1, b=1/2 (Wait, check math again). Solving properly yields a/b = 60.

Let X = {x in N : 1 <= x <= 19} and for some a, b in R, Y = {ax + b : x in X}. If the mean and variance of the elements of Y are 30 and 750, respectively, then the sum of all possible values of b is

A. 20
B. 80
C. 100
D. 60 ✓

Answer: D. 60
Mean(Y) = a*Mean(X) + b. For X = {1..19}, Mean = 10, Variance = (19^2-1)/12 = 30. Given Mean(Y)=30, 10a+b=30. Given Variance(Y)=a^2*Var(X)=750, so a^2*30=750, a= +/- 5. If a=5, b=-20. If a=-5, b=80. Sum of possible b = 80 - 20 = 60.

Let the mean and variance of 12 observations be $\frac{9}{2}$ and $4$ respectively. Later on, it was observed that two observations were considered as 9 and 10 instead of 7 and 14 respectively. If the correct variance is $\frac{m}{n}$, where $m$ and $n$ are coprime, then $m+n$ is equal to

A. 314
B. 315
C. 316
D. 317 ✓

Answer: D. 317
For $n=12$, the incorrect mean is $\bar x=\frac{9}{2}$, so the incorrect sum is $$\Sigma x = 12\cdot \frac{9}{2}=54.$$ The variance formula gives $$\sigma^2=\frac{\Sigma x^2}{n}-\bar x^2=4,$$ so $$\frac{\Sigma x^2}{12}=4+\left(\frac{9}{2}\right)^2=4+\frac{81}{4}=\frac{97}{4}$$ which gives $$\Sigma x^2=291.$$ Now correct the two mistaken observations: values $9,10$ were used instead of $7,14$. So the corrected sum is $$54-9-10+7+14=56,$$ and corrected sum of squares is $$291-9^2-10^2+7^2+14^2=291-81-100+49+196=355.$$ Hence corrected variance is $$\frac{355}{12}-\left(\frac{56}{12}\right)^2=\frac{355}{12}-\frac{196}{9}= \frac{1065-784}{36}=\frac{281}{36}.$$ Thus $m+n=281+36=317$.

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