To think that this sum is super simple

[PeerieBreeks]

and can't understand how so many people on Facebook have it so completely wrong (and can justify it to themselves).

Without adding your reasoning, tell me what you think the answer is.

Does no-one care about the poor bloody horse in all this? He won't know if he's coming or going!

$20 by the way.

I'll give you the qualified accountants answer.

How much do you want him to have made?

I just need to find some goats, cars and doors (plus a teenager to be the show host..), and I'll try this out, and see just how many cars I win...

That was always the part I struggled with. People would explain that your original odds haven't changed, but I would think "they have, though, because now you know your door is one of only two possibilities".

But the remaining two doors aren't equal - you chose yours at random, whereas the one left is the "best" choice of the remaining 2/3.
I've seen it explained this way before: imagine you are playing against the host, and there are 100 doors. You pick a door at random, then the host looks behind all the other doors and picks one. The others are all opened to reveal goats (as expected, since either you have the car or the host picked the winning door) leaving your two choices. Who has the greater chance of having the winning door now, and would you switch with the host given the chance?

Or if there are 3 party bags, and 3 DC. 2 bags have a single sweet, and the 3rd has a chocolate egg. The DC are told to close their eyes and pick a bag at random. Dc1 does so first, then DC2 peeps in both bags, picks one and gives the remaining bag to DC3.
DC1 has a 1/3 chance of having the better bag. DC3 has a 0/3 chance - either DC1 picked it at random or DC2 knowingly picked the better one. DC2 has a 2/3 chance, since they got to look through the 2 bags and pick the best 1.
So if DC3's bag is opened to reveal a sweet/goat, and DC1 is given the chance to swap with DC2, what should they do?

Started with 60, made transactions, now has 90. Profit 30.

$10 profit

I don't understand people saying $20?

I don’t understand how people are getting anything other than 20 😐

He bough for $60 sold for $70 thus making $10 then bought the same house for $80 meaning he had to put the $70 to it plus an extra $10 dollars to it then sold it for $90 which means he broke even.

He made a profit of $10 for each horse therefore he made $20

I like this. While I know it is $20, I can sort of convince myself it is $10, but I can't quite work out what the logical mistake I'm making.

What I would say is that if you can't see how it might be something other than $20, you're not thinking enough

I got this wrong first of all.

Incorrect logic:

I buy it for $60 and sell for $70, gaining $10.

I sell for $70 and buy back for $80, losing $10, so I'm back to zero.

I buy for $80 and sell for $90, gaining $10, meaning I'm up $10 overall.

The thing is, I know this is wrong, but I'm not entirely sure why. But I think it might be that I basically count the same transactions twice?

A correct approach is to consider each transaction individually in turn:

I start with $0.

I pay $60 so I'm -$60.

I sell for $70 so I'm +$10.

I pay $80 so I'm -$70.

I sell for $90 so at the end of it all I'm +$20.

How can he break even when he makes two lots of 10 dollars profit??

Up $10

Who says he starts with nothing??

Let’s say you start with 100.

You buy a horse for 60 and sell it for 70. You now have 110.

Then you buy it for 80 and sell it for 90. You now have 120. You made 20 profit.

Yeah, actually, re-reading what I wrote, it's very obvious that in the incorrect logic I count the sell-for-70 and buy-for-80 stages twice over, leading to the wrong answer.

He made 20 dollars.

To me he's only made $10 as the first time he was up $10 he blew by buying the same horse back for more, therefore canceling out his profit (on the same horse) so he only makes $10 overall.

$30

$10 up, but I’m shit at maths. How are people getting $20

Who got the horse I want to know

Strong - there are many posts on here explaining how you get 20. He buys a horse and sells it for 10 more dollars. Then he buys a horse and sells it for ten more dollars. That it's the same horse is irrelevant. Total profit twenty

Groundhog Day. If you read a few pages of the thread all will become clear

Not RTFT but 10$

"Who says he starts with nothing??"

How much he starts with is irrelevant. When you do

-60
+70
-80
+90

he is always left with 20 more than he started with.

I guess the reason people are getting it wrong is because they aren't approaching it from a logical mathematic angle.

–60 + 70 –80 + 90 = $20

But how much did he pay for delivery each time, how many days work did he lose waiting in for the courier, and how much hay/oats did the horse eat while it was in his possession?

These things will undoubtedly cost him more than $20 so in reality he's in the hole.