Question:medium

If the mean and variance of a Binomial variate \(X\) are 2 and 1 respectively, then the probability that \(X\) takes a value greater than 1 is:

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For Binomial distributions, use \(P(X > k) = 1 - P(X \leq k)\).
Updated On: Nov 26, 2025
  • \(\frac{2}{3}\)
  • \(\frac{4}{3}\)
  • \(\frac{7}{8}\)
  • \(\frac{11}{16}\)
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The Correct Option is D

Solution and Explanation

Given a binomial distribution with mean \(\mu = 2\) and variance \(\sigma^2 = 1\), we have:
\[ \mu = np = 2 \] and \[ \sigma^2 = np(1-p) = 1. \] From \(\mu = np = 2\), we derive \(p = \frac{2}{n}\). Substituting this into the variance equation yields:
\[ n \cdot \frac{2}{n} \cdot \left( 1 - \frac{2}{n} \right) = 1. \] Simplifying this equation leads to:
\[ 2 \left( 1 - \frac{2}{n} \right) = 1 \quad \Rightarrow \quad 2 - \frac{4}{n} = 1 \quad \Rightarrow \quad \frac{4}{n} = 1 \quad \Rightarrow \quad n = 4. \] Substituting \(n = 4\) back into \(p = \frac{2}{n}\) gives:
\[ p = \frac{2}{4} = \frac{1}{2}. \] We now need to compute \(P(X>1)\). This can be expressed as:
\[ P(X>1) = 1 - P(X \leq 1). \] For a binomial distribution \(X \sim B(4, \frac{1}{2})\), \(P(X \leq 1)\) is calculated as:
\[ P(X \leq 1) = P(X = 0) + P(X = 1). \] Using the binomial probability mass function \(P(X = k) = \binom{n}{k} p^k (1-p)^{n-k}\), we find:
\[ P(X = 0) = \binom{4}{0} \left(\frac{1}{2}\right)^0 \left(\frac{1}{2}\right)^4 = \frac{1}{16} \] and \[ P(X = 1) = \binom{4}{1} \left(\frac{1}{2}\right)^1 \left(\frac{1}{2}\right)^3 = \frac{4}{16}. \] Therefore, \[ P(X \leq 1) = \frac{1}{16} + \frac{4}{16} = \frac{5}{16}. \] Finally, the probability \(P(X>1)\) is:
\[ P(X>1) = 1 - \frac{5}{16} = \frac{11}{16}. \] The probability that \(X\) is greater than 1 is \(\frac{11}{16}\).
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