December 2007 LSAT Section 1 Question 21

The trees always blossom in May if April rainfall exceeds 5 centimeters. If April rainfall exceeds 5 centimeters, the...

maonuo on August 19, 2020

Please Help

I'm having a hard time seeing a flaw in this argument. If we combine all the premises using the transitive property it should be: tree blossoms-->more than 5-->Reservoir full. So reversing and negating this statement you can conclude that if reservoir not full-->tree blossoms. Also, I'm not sure why A is the correct answer if you could also help with explaining this I would appreciate it. Thanks.

Create a free account to read and take part in forum discussions.

Already have an account? log in

shunhe on August 20, 2020

Hi @maonuo,

Thanks for the question! So that’s not actually how we would link the premises together. Let’s diagram this premise by premise. First, we’re told that if April rainfall exceeds 5cm, then the trees blossom in May. So this is a simple if-then statement, or

AR > 5 cm —> TB in May

Next sentence: if April rainfall exceeds 5 cm, then the reservoirs are always full on May 1. So this is

AR > 5 cm —> RF May 1

Now we’re told that the reservoirs weren’t full on may first. Well, that means

~RF May 1

So we can get from the contrapositive of the second statement

~RF May 1 —> ~AR > 5 cm

So we know that the rainfall in April didn’t exceed 5 cm. But can we then use that to conclude that the trees didn’t blossom in May? No, that’s a mistaken negation! Remember, we can’t go

A —> B?~A
Therefore: ~B

And so that’s what the flaw here is, and that’s the one mirrored in answer choice A.

Garlic in pantry —> Fresh
Garlic in pantry —> Potatoes on basement stairs
~Potatoes not on basement stairs
Therefore: ~Fresh

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