Answer:
0.1655
Explanation:
This is a binomial probability.
THe formula is:

Where n is the total number of people
r is how many we want (we want 1/3rd of 18 which is 6, r= 6)
p is probability of success (0.4)
Putting the information, we get:

Thus, the probability that exactly a third of the buyers would prefer brown is 0.1655