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Explanation:
There are A = 13 hearts and B = 4 copies of "7"
The count A+B = 13+4 = 17 may seem like it's the count of hearts or "7" cards, but we're double counting the 7 of hearts. So the true count is actually 17-1 = 16
There are 16 cards we want (either a heart or a "7") out of 52 total. Therefore, the probability is 16/52 = (4*4)/(4*13) = 4/13
