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"Cafeteria Multiplication"
Two intelligent, honest students are sitting together at lunch one
day when their math teacher hands them each a card. “Your cards each
have an integer on them,” the teacher tells them. “The product of the
two numbers is either 12, 15 or 18. The first to correctly guess the
number on the other’s card wins.”
The first student looks at her card and says, “I don’t know what your
number is.”
The second student looks at her card and says, “I don’t know what
your number is, either.”
The first student then says, “Now I know your number.”
What number is on the loser’s card?