Top Law Schools

I'm having trouble diagramming rule #4.
"If it is not the case that the park contains both laurels and oaks, then it contains firs and spruces"

Some on this board had this question, but I'm still not sure why it's like this:
~(L + O) --> F + S
contra: ~F/~S --> L + O

Is it because "laurels and oaks" is treated as one unit?
Initally I diagrammed it this way:
~L and ~O --> F and S
contra: ~F or ~S --> L or O
I know it's wrong, I'm not sure, how and why "laurels and oaks" is being treated as one unit. Is it because of the keyword "both" ?

Since it only takes one of L and O being out to trigger the rule, they should be joined by "or" in the original version:
~L or ~O → F and S
~F or ~S → L and O (contrapositive)

Essentially, the rule tells us that the park must contain at least one of the two pairs (LO and FS).

Cambridge LSAT wrote:Since it only takes one of L and O being out to trigger the rule, they should be joined by "or" in the original version:
~L or ~O → F and S
~F or ~S → L and O (contrapositive)

Essentially, the rule tells us that the park must contain at least one of the two pairs (LO and FS).

I see, that was the mistake, thank you.

this game is tough. For instance, Rule 2 says if F --- > ~P, so, I was thinking the contrapositive of that would be P---> ~F. Which lead to my confusion regarding problem 10, which says If F is out, what MUST be true. So, I'm thinking, F is out, that must mean P is in, which is the answer for (D). However, that is the wrong answer. Ugh . . . .