People who are red/green color-blind cannot distinguish between green and brown. Gerald cannot distinguish between gr...

SamA June 4, 2020

Hello @rosemarie,

Maybe I can help. These drills may seem a bit abstract, but they are quite helpful for your understanding of sufficient and necessary reasoning. You will have to follow the same procedures on real Logic Games and Logical reasoning questions.

Let's start with argument completion. The goal here is to connect different pieces of information in order to draw a conclusion. Try to look for common terms in order to make these connections. I'll give you a simple example.

P: A ---> B
P: B ---> C
C: ?

What do these premises have in common? B. Because of this, we can connect them.

A ---> B ---> C
Because you watched the lecture, you should know that we can conclude:
A ---> C

This is the answer! We have connected two conditions (A and C), that were not explicitly connected in the premises. No matter how complicated these become, this goal stays the same. I'll give you another example.

P: not B ---> A
P: B ---> C
C:

At first glance, we do not have common terms. However, we can create them by using a contrapositive. Remember that a contrapositive represents the exact same logic as the original statement, so they are completely interchangeable. This is why there are two correct answers on the flashcards. I'll take the contrapositive of the first premise.

P: not A ---> B
P: B ---> C

Now we can make a connection!
not A ---> B ---> C

Our answer:
not A ---> C
or
not C ---> A

The flashcard answers are going to require you to use every single premise, so don't leave any out when you get 3-premise questions.

Let's talk about missing premise drills. Here, we are given the conclusion.

P: Y ---> not X
P: ?
C: X ---> not Z

I like to think about it this way. Take the starting point of the conclusion (X) and ask yourself, how can I get to the end (not Z) using the premises? You will have to create a premise in order to get there. It's like making a path from X to not Z.

So, we will start with X. But I don't see one in our premise, so I need to use a contrapositive.

P: X ---> not Y
P: ?
C: X ---> not Z

We now know that X leads to not Y. How can we use this to get to not Z? We just have to write it!

P: X ---> not Y
P: not Y ---> not Z
C: X ---> not Z

For both of these drills, we want to look for common terms. When in doubt, make contrapositives. I hope this helps.