Which one of the following could be a complete and accurate list of the friends who appear together in a photograph?

Taylor on May 28 at 06:48PM

Confused on game set-up

I watched the game-set up video, and I am confused why the instructor sets rule 1 as s-->w. Would W not be sufficient for S? I read it as if W is in the photograph then we know S is in the photograph. This then carried to rule 2, which probably led to me not doing well on this game.

2 Replies

Shunhe on May 28 at 10:11PM

Hi @T.Neill,

Thanks for the question! So let’s take a look at the first rule, which reads “Wendy appears in every photograph that Selma appears in.” How can we translate this into conditional language? Well, what this is really telling us is that if S is in a photo, then W is in that photo, because W is in every photo that S is in. But translating this, we get


It doesn’t mean the other way around though. Let’s say that W is in a photo. Does that mean that S is in it? No, not necessarily. To know that, we’d have to be told that S is in every photo that W is in. But we’re not told that, we’re told that W is in every photo S is in. And so we can’t get W—>S from the first rule. Same goes for rules 2 and 3. This is definitely an important concept to master, as you’ve learned from misdiagramming this game.

Hope this helps! Feel free to ask any other questions that you might have.

Brett on August 23 at 08:59AM

I spent about 30 seconds trying to figure out the directionality for that one, too, until I noticed the word "EVERY," which introduces the sufficient clause. Rewritten, it states

EVERY photo Selma appears in, Wendy appears in:
S --> W