That wall is supported by several joists. The only thing that can have caused the bulge that the wall now has is a br...

Naz on June 5, 2014

First let's diagram the stimulus.

We are told that a wall is supported by several joists.

The second sentence states: "The only thing that can have caused the bulge that the wall now has is a broken joist."

This statement give us a Sufficient & Necessary statement and a premise. We know that "the only" introduces a sufficient condition. So we can rewrite this: if bulge, then there is a broken joist.

B ==> BJ
not BJ ==> not B

We also know that "the wall now has" a bulge.

The passage concludes that at least one of the joists is, therefore, broken.

So, this argument gives us a Sufficient & Necessary statement to follow: if there is a bulge, then there is a broken joist. We know there is a bulge. Thus, according to the Sufficient & Necessary statement, there is a broken joist. So the conclusion is valid (i.e. positive argument structure).

Answer choice (A) follows this same pattern. We know that nothing other than one of the players in the orchestra making a mistake could have made the conductor grimace in the way she did. So, if the conductor grimaced, then it was because one of the players in the orchestra made a mistake.

G ==> PMM
not PMM ==> not G

We know that the conductor just grimaced.

The answer choice concludes: "At least one of the players in the orchestra must have made a mistake."

This follows the Sufficient & Necessary statement given. We know the conductor grimaced, so it follows that one of the players must have made a mistake. This is also a valid positive argument just like the one we saw in the stimulus.

Hope that was helpful! Let us know if you have any other questions.

Batman on June 8, 2014

Wow!!!lol
Thanks a lot!!! ^^