opponent wrote:A mocktail has to be prepared from 3 kinds of whisky, 4 kinds of soda, and 5 kinds of fruit juice. How many mocktails can be made taking at least one of each other kind?

Number of ways to select at least one object from n different objects = (2^n - 1)

Therefore,
Number of ways we can select at least one whiskey from 3 kinds of whiskey = (2^3 - 1) = 7
Number of ways we can select at least one soda from 4 kinds of soda = (2^4 - 1) = 15
Number of ways we can select at least one fruit juice from 5 kinds of fruit juice = (2^5 - 1) = 15

Hence, total number of possible combinations = 7*15*31 = 3255