Question

Use an Euler diagram to determine whether the argument is valid or invalid.

1) All students who study get better grades
Roger is a student who studies
Roger will get better grades


2) All tigers are felines
All felines are mammals
All mammals nurse their young
All tigers nurse their young

18) Some investments are risky
Real state is an investment
Real state is risky

Answer 1

So, Eulers Diagram is a quick way for me to determine that one statement is a fallacy by representing the premises on a circle.
I'll give an example with the third option:
Let's first draw a circle that represents the "investments":

Inside of it, we have to draw another circle indicating "risky investments" because the premise states "SOME investments are risky".
So we will divide it into those who are risky and those who aren't:

Now, here's the catch, because there are two possiblities, that the empire state can be here:

or here:

So, since it can be on two places I conclude that the third option is a fallacy.