Day 12: Picking a Christmas Card

Rachel has found that her class' Christmas cards have all been mixed up in a pile. There are 33 cards in the pile, and she knows that there are nn cards that are addressed to her.

She picks two cards at random (without replacement). The probability that both cards are addressed to her is 18\frac18.

Find the value of nn.


This problem is part of the Advent Calendar 2015.
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