Answer:
For this case we can use the probability mass function and we got:

Step-by-step explanation:
Previous concepts
A Bernoulli trial is "a random experiment with exactly two possible outcomes, "success" and "failure", in which the probability of success is the same every time the experiment is conducted". And this experiment is a particular case of the binomial experiment.
The binomial distribution is a "DISCRETE probability distribution that summarizes the probability that a value will take one of two independent values under a given set of parameters. The assumptions for the binomial distribution are that there is only one outcome for each trial, each trial has the same probability of success, and each trial is mutually exclusive, or independent of each other".
The probability mass function for the Binomial distribution is given as:
Where (nCx) means combinatory and it's given by this formula:
Solution to the problem
Let X the random variable of interest, on this case we now that:
For this case we can use the probability mass function and we got:

Step-by-step explanation:
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(A) The customer that owns a 3-kW system exports 120 kWh monthly to the grid will have the following bills for both companies:
For ZG&E that collects $3 per kWh on a monthly basis, the monthly bill will be

For Ready Edison that collects $0.06 per kWh exported energy monthly, the computation of bill will be

(B) The customer that owns a 5-kW system exports 300 kWh monthly to the grid will have the following bills for both companies:
For ZG&E that collects $3 per kWh on a monthly basis, the monthly bill will be

For Ready Edison that collects $0.06 per kWh exported energy monthly, the computation of bill will be