- #1
eprparadox
- 138
- 2
Homework Statement
Show that the expected number of 5's in n tosses of a die is n/6.
Homework Equations
x = random variable representing the number of 5's
[tex]E\left( x\right) =\sum _{i}p_{i}x_{i}[/tex]
The Attempt at a Solution
The probability of getting i 5's in n tosses is:
[tex]\left( \dfrac {1} {6}\right) ^{i}\left( \dfrac {5} {6}\right) ^{n-i}[/tex]
So the expectation value of x is:
[tex]E\left( x\right) =\sum _{i=0}^{n}i\left( \dfrac {1} {6}\right) ^{i}\cdot \left( \dfrac {5} {6}\right) ^{n-i}[/tex]
I typed this into wolframalpha and I don't get n/6 as I thought I would.
I'm not sure where I went wrong. Any ideas would be greatly appreciated. Thanks!