odds calculation question

[joe2004]

In Sklansky's advanced hold em book there is a probablity table in back I do not understand. He calculates the probablity of completing a four-flush post-flop in hold em at about 35%. In other words, with two cards to come...
my understanding of the math is 9/47 + 9/46 or about 38.7%. What is the math used to come up with Sklansky's figure? Why is it wrong to add the percentages together for the turn and river cards? [img]/images/graemlins/confused.gif[/img]