# Conditional Expectation Help

• Dec 8th 2010, 04:59 PM
theprestige
Conditional Expectation Help
Suppose you flip one fair coin and roll one fair six-sided die. Let X be the number showing on the die, and let Y = 1 if the coin is heads with Y = 0 if the coin is tails. Let Z = XY.

Compute E(Z):

Am I correct when I do E(Z) = E(X)E(Y) = (3.5)(1/2) = 7/4

Also, how do i do this?

Compute E(Z|X = 4)

Thanks
• Dec 9th 2010, 04:52 AM
Focus
Quote:

Originally Posted by theprestige
Suppose you flip one fair coin and roll one fair six-sided die. Let X be the number showing on the die, and let Y = 1 if the coin is heads with Y = 0 if the coin is tails. Let Z = XY.

Compute E(Z):

Am I correct when I do E(Z) = E(X)E(Y) = (3.5)(1/2) = 7/4

I am guessing X and Y are independent, in which case you are correct.

Quote:

Also, how do i do this?

Compute E(Z|X = 4)

Thanks
Given that you rolled a 4, what is the expectation of Z? There are only 2 values Y could take with equal probability, so look at what happens at each. You could look at this as $\mathbb{E}[4Y]$.
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