Question 1191145
Here's how to find the mean of the random variable Z for each case:

**Key Principle:**

The mean of a linear combination of random variables is equal to the linear combination of their means.  That is:

E(aX + bY) = aE(X) + bE(Y)

Where 'a' and 'b' are constants.

We are given E(X) = 20 and E(Y) = 40.

**A) Z = 25 - (1/2)X**

E(Z) = E(25 - (1/2)X)
E(Z) = 25 - (1/2)E(X)
E(Z) = 25 - (1/2)(20)
E(Z) = 25 - 10
E(Z) = 15

**B) Z = (1/3)X - 8**

E(Z) = E((1/3)X - 8)
E(Z) = (1/3)E(X) - 8
E(Z) = (1/3)(20) - 8
E(Z) = (20/3) - (24/3)
E(Z) = -4/3  or -1.33 (approximately)

**C) Z = X + Y**

E(Z) = E(X + Y)
E(Z) = E(X) + E(Y)
E(Z) = 20 + 40
E(Z) = 60

**D) Z = X - Y**

E(Z) = E(X - Y)
E(Z) = E(X) - E(Y)
E(Z) = 20 - 40
E(Z) = -20

**E) Z = -3X + 3Y**

E(Z) = E(-3X + 3Y)
E(Z) = -3E(X) + 3E(Y)
E(Z) = -3(20) + 3(40)
E(Z) = -60 + 120
E(Z) = 60