Answer:
a) There is a 1.21% probability that both contain diet soda.
b) There is a 79.21% probability that both contain diet soda.
c) 
is unusual, 
is not unusual
d) There is a 19.58% probability that exactly one is diet and exactly one is regular.
Step-by-step explanation:
There are only two possible outcomes. Either the can has diet soda, or it hasn't. So we use the binomial probability distribution.
Binomial probability distribution
The binomial probability is the probability of exactly x successes on n repeated trials, and X can only have two outcomes.
In which 
is the number of different combinatios of x objects from a set of n elements, given by the following formula.
And 
is the probability of X happening.
A number of sucesses 
is considered unusually low if 
and unusually high if 
In this problem, we have that:
Two cans are randomly chosen, so 
Two out of 18 cans are filled with diet coke, so 
a) Determine the probability that both contain diet soda. P(both diet soda)
That is P(X = 2).
There is a 1.21% probability that both contain diet soda.
b)Determine the probability that both contain regular soda. P(both regular)
That is P(X = 0).
There is a 79.21% probability that both contain diet soda.
c) Would this be unusual?
We have that is unusual, since 
For P(X = 0), it is not unusually high nor unusually low.
d) Determine the probability that exactly one is diet and exactly one is regular. P(one diet and one regular)
That is P(X = 1).
There is a 19.58% probability that exactly one is diet and exactly one is regular.
