John and Gerry, who are walking along with a clearly visible number written on their foreheads, 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). So they know enough, even more than enough (!), in order to solve the riddle.

The primises in the riddle “Perfect Logicians” are enough too. Again:

A and B know enough in order to solve the riddle.

If it is said that two humans see a number, then we can surely assume that this humans are capable of seeing and reading, and of knowing what they see and read. That is common sense.