Destroyer QR Question 20 & 27

[bruinguy083]

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I'm really confused on probability. Some questions are really and some are really difficult for me. In Destroyer, I don't see the difference in questions 20 & 27. they seem to be asking the same things, and the verbal sol'n is the same, yet the formula is different.

Can someone explain the difference in solutions?

 

[Streetwolf]

Posted 50 billion times. Do a search or look back 2, 3, or 4 pages. You'll find what you want.