Answer:
a) E(x)=2.5 V(x)=1.59
b) E(R)=17 V(R)=6.36
c) The probability that, when the four make their orders, everyone gets what they ordered is P=0.66.
Explanation:
a) The mean E(x) can be calculated as:

The variance V(x) can be calculated as:

b) The revenue can be expressed as:

Then, the expected value of R canbe expressed in function of x:

The expected revenue is $17.
The variance of R is

c) In the case there are only 3 Cokes and 3 Sprites available, the only orders that can not be fullfilled are when X=0 (they order 4 Sprites) and X=4 (they order 4 Cokes).
The probability of these events is:

So, the probability of everyoned getting what they ordered is:

The probability that, when the four make their orders, everyone gets what they ordered is P=0.66.