To find the probability of a person buying more than one costume, divide the number of people that has bought more than one costume over the total amount of people that have visited the website.
The total amount of people that have visited the website is given by the following sum:
Since 16 visitors have purchased more than one costume, divide 16 over 338:
Since 16/338 is lower than 0.05 but greater than 0.04 and it's closer to 0.05, then, the probability of the next person purchasing more than one costume, rounded to the nearest hundredth, is 0.05.