1. Expectation

I am wondering if I could get help on the following HW problem:
3 trails each have the same probability of success. If X is the total # of successes and if E[X]=1.8. What is the smallest possible value of P(X=3)?
I know that the answer is 0 but I do not know why. I'll appreciate any help.

2. Can you post the exact question?

3. Originally Posted by thomas_donald
I am wondering if I could get help on the following HW problem:
3 trails each have the same probability of success. If X is the total # of successes and if E[X]=1.8. What is the smallest possible value of P(X=3)?
I know that the answer is 0 but I do not know why. I'll appreciate any help.
Let $\displaystyle p$ be the probability of success on a single trial. Then:

$\displaystyle E(X)=0.P(x=0)+1.P(x=1)+2.P(x=2)+3.P(x=3)=1.8\ \ \ \ ...(1)$

where:

$\displaystyle P(x=0)=(1-p)^3$,

$\displaystyle P(x=1)=3.(1-p)^2.p$,

$\displaystyle P(x=2)=3.(1-p).p^2$,

$\displaystyle P(x=3)=p^3$.

Substituting these into the equation $\displaystyle (1)$ gives a cubic in $\displaystyle p$ and the question wants to know the smallest positive root of this cubic.

CB

4. Thanks a lot for the help.
This is the exact question and the trails are not independent.