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CBSE Class 12 Math 2018 Solved Paper

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Question : 23 of 29
Marks: +1, -0
Two numbers are selected at random (without replacement) from the first five positive integers. Let X denote the larger of the two numbers obtained. Find the mean and variance of X.
Solution:  
Given that two positive integers can be selected from the first 5 positive integers without replacement in 5 × 4 = 20 ways.
X represents the larger of the two numbers obtained.
Hence, X can be 2, 3, 4, 5.
For x = 2, possibilities are (1, 2) and (2, 1).
∴ P (X = 2) = 220\frac{2}{20}
For X 3, possibilities are (1 , 3) , (2 , 3) , (3 , 1) , (3 , 2)
∴ P (X = 3) = 420\frac{4}{20}
For X = 4, possibilities are (1 , 4) , (2 , 4) , (3 , 4) , (4 , 3) , (4 , 2) , (4 , 1)
∴ P (X = 4) = 620\frac{6}{20}
For X = 5, possibilities are (1 , 5) , (2 , 5) , (3 , 5) , (4 , 5) , (5 , 4) , (5 , 3) , (5 , 2) , (5 , 1)
∴ P (X = 5) = 820\frac{8}{20}
X P (X) X P (X) X2X^2 P (X)
2220\frac{2}{20} 420\frac{4}{20} 820\frac{8}{20}
3 420\frac{4}{20}1220\frac{12}{20} 3620\frac{36}{20}
4 620\frac{6}{20} 2420\frac{24}{20}9620\frac{96}{20}
5 820\frac{8}{20} 4020\frac{40}{20} 20020\frac{200}{20}
Total Σ x P (X) = 4 Σ x2x^2 P (X) = 4
⇒ E (X) = Σ x P (x) = 4 and E (X2)(X^2) = Σ x2x^2 P (X) = 17
⇒ V (X) = E (X2)(X^2) - [E(X)]2[E(X)]^2
⇒ V (X) = 17 - [4]2[4]^2
⇒ V (X) = 1
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