Smart

edit, the guy who sold the box actually lost $10, while the guy that bought the box gained $10

It’s the other way around. The seller gained 10 and the buyer lost 10

wait yeah you're right, the buyer put $20 into the box, then bought it from the seller for $30 dollars, with the box having $40 into it, 40-50 equals -10, the seller put $20 into the box, then sold the box for $30, 30-20 equals +10

The seller makes $10 profit from his original $20 , but he loses $10 from the $40 in the box. The buyer gains $10 from the $40 that he bought for $30, but he also loses $10 because he actually spent $50 from the first $20. So, because the first guy spent $20 and got $30, he gets $10 profit. The second guy spent $50 and got $40, so he lost $10.

I had to think way too hard for that, it hurt my brain and my brother’s.

After putting they each put $20 in the box, the are both at -$20

Then after buying, the dude who bought the box is at -$50 and the other guy is at $10

Then when the dude opens the box he gets $40 and is at -$10

my brain knowing its wrong but not how

I'm sorry, but this was making me really angry for no reason whatsoever and I need to suck all the fun out of it

Scenario Breakdown:
Initial Contribution:

You put $20 in the box.
I put $20 in the box.
Total in the box: $40.
Transaction:

You sell me the box for $30.
I give you $30.
Analyzing the Transaction:
Before the Sale:
You:

You have $20 less because you put it in the box.
Your balance: -$20.
Me:

I have $20 less because I put it in the box.
My balance: -$20.
During the Sale:
You:

You receive $30 from me.
Your balance: $30 - $20 (initial contribution) = $10 profit (so far).
Me:

I give you $30.
My balance: -$20 (initial contribution) - $30 = -$50.
However, I now own the box which contains $40.
After the Sale:
You:

You have $30 (received from me) but you initially put in $20.
Your net profit: $30 (from sale) - $20 (initial contribution) = $10 profit.
Me:

I have the box with $40 in it, but I spent $30 to buy it.
Net balance: $40 (in the box) - $30 (paid to you) - $20 (initial contribution) = -$10.
Conclusion:
You indeed made a $10 profit.
However, I did not make a profit. I actually have a net loss of $10.
Misconception:
The statement "we both make a $10 profit" is incorrect. Here's why:

Your Perspective:

You received $30 for the box and initially invested $20.
Your net profit is $10.
My Perspective:

I now have the box with $40 in it.
I spent $30 to buy the box and initially invested $20.
My net balance is actually a $10 loss (not a profit).

I probably just made a 30 minute rant for nothing. Either way, if you read all of this, congratulations.

The math ain't mathing