Alex, Betty and Craig don't want to disclose their weekly income to each other, but still wish to find some information about their wages.
They play the following game: each person takes two pieces of paper. On the first piece of paper they take their own salary and add some nice round number to it. On the second piece of paper they take their own salary and subtract the same nice round number from it. The nice round number probably varies from person to person, but since they don't talk about it, we can't say for sure.
They obtain the following result in thousands dollars: 1, 2, 3, 4, 5, and 6.
What do they know now?