If a solution is possible, then they have to know a certain number range, thus the upper limit and the lower limit of the range of their numbers, and they have to know another aspect, for example: a possible sum, a possible difference, a possible product, a. possible quotient of their two numbers (for instance: Gerry only knows a sum of the two numbers of a certain number range, whereas John only knows a product of the two numbers of the same certain number range).

If a solution is not possible, then they have to know how to order two beers.

In any case, ordering two beers could actually happen. Couldn’t it?