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One of the rules is "neither oils nor pastels can be scheduled for the same day as lithography."

To me, that means two things: If L not O AND If L not P.

What is the contrapositive for that? I am trying to do question 15 and the answer is D, pastels, but I don't see why Pastels can't go with Oils on day 2. I know pastels can't go on day 1, and they can't go with L on day 3, but why not with O on day 2?

I hope I explained that right.

Thanks.

One more thing, "if and only if" and "but only if" are the same, and they tie the two things together, always, right?

This is actually game 3. You can note the third condition in a number of different ways:
O --> ~L; L --> ~O (contrapositive)
P --> ~L; L --> ~P (contrapositive)

Alternatively, you can use double-not arrows:
O <--|--> L
P <--|--> L

Assuming your diagram has the days listed horizontally, and AM/PM listed vertically, you could also make an oval with O above L and put a line through it. You could do the same with P and L.

#15) P cannot be scheduled for either of the first two days due to the fourth condition, and it cannot be scheduled for the third day due to the third condition.

Yes, "X if and only if Y" is equivalent to "X if but only if Y."