There are 11 flush cards and 39 non-flush cards, and there are C(50,3) possible flops ignoring order.
no spades: C(39,3) / C(50,3) = 0.46627551
1 spade: 11*C(39,2) / C(50,3) = 0.415867347
2 spades: C(11,2)*39 / C(50,3) = 0.109438776
3 spades: C(11,3) / C(50,3) = 0.008418367
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total = 1
I'll post a review of Petriv's book once I've finished examining it. Here are some earlier
comments I have made about it.