Simple...
you have:

and

To solve for....

Multiply to remove the denominator-->>

n=12*3
n=36
Then-->>>

Do the same thing-->>

n=-12*3
n=-36
Thus, your answer.
I believe it would be the last answer. All you have to do is the pencil test to see if its negative or positive slope.
Answer:
4 friends, 40$, so each had 10$
40/4=10-7.5= 2.5
Each had $2.50 left