X is a binomial variable such that 2 P(X = 2) = P( x = 2) and mean of ...
Given:
- 2 P(X = 2) = P( x = 2)
- Mean of X = 10/3
- To find: P(X ≤ 2)
Solution:
Let n be the number of trials and p be the probability of success in each trial. Then, from the given information, we have:
- E(X) = np = 10/3
- 2P(X = 2) = P(X = 2)
Using the formula for the mean of a binomial distribution, we get:
np = E(X) = (2P(X = 2) + 2P(X > 2))n
Simplifying this equation using the given information, we get:
(4/3) = (2P(X > 2))n
Also, we know that the probability of X assuming at most the value 2 is:
P(X ≤ 2) = P(X = 0) + P(X = 1) + P(X = 2)
Using the formula for the binomial distribution, we get:
P(X = k) = (n choose k) * p^k * (1-p)^(n-k)
Substituting the given information, we get:
P(X ≤ 2) = (n choose 0) * p^0 * (1-p)^n + (n choose 1) * p^1 * (1-p)^(n-1) + (n choose 2) * p^2 * (1-p)^(n-2)
= (1-p)^n + np(1-p)^(n-1) + (n(n-1)/2)p^2 (1-p)^(n-2)
Substituting the value of (4/3) for np in the above equation, we get:
P(X ≤ 2) = (1-p)^n + 4/3(1-p)^(n-1) + (n(n-1)/2)(4/9)p^2 (1-p)^(n-2)
We need to solve for p and n in terms of the given information. One way to do this is to use the fact that 2P(X = 2) = P(X = 2), which implies:
2(n choose 2) * p^2 * (1-p)^(n-2) = (n choose 2) * p^2 * (1-p)^(n-2)
Simplifying this equation, we get:
2(n-1)(n-2)p^2 = (n-2)(n-1)p^2
Dividing both sides by (n-1)(n-2)p^2, we get:
2 = 1/(n-2)
Solving for n, we get n = 3. Substituting this value into the equation (4/3) = (2P(X > 2))n, we get:
P(X > 2) = 1/3
Substituting the values of p and n into the expression for P(X ≤ 2), we get:
P(X ≤ 2) = (1-p)^n + np(1-p)^(n-1) + (n(n-1)/2)p^2 (1-p)^(n-2)
= (1-1