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.

From a math forum:

Here's where the mistake lies. Let's assume that Bill (the guy buying and selling the horses) has $60 at the beginning of this little horse- trading. So he spends it on the horse. Then he gets $70 for it. If he wants to buy it again for $80, you're right that he has to borrow $10 from someone else (or from himself - perhaps from he had some other money he had planned to use for another purchase), but we have to keep track of this debt. See, he's not even now, he actually has -10 dollars. So then he gets $90 back for the horse. But since he had -$10, the net effect of getting that $90 is to make him have 80 dollars (you might think about that as paying back the $10 he owed). So since he started out with $60, and he has $80 at the end, the overall effect was to gain $20.

He made $10.

...Or $20.

Just add together the money he spent and then the money he received and take one away from the other. £20 profit.

(My DC tells me this is like the year 5 maths questions)

This explains it

Here's where the mistake lies. Let's assume that Bill (the guy buying and selling the horses) has $60 at the beginning of this little horse- trading. So he spends it on the horse. Then he gets $70 for it. If he wants to buy it again for $80, you're right that he has to borrow $10 from someone else (or from himself - perhaps from he had some other money he had planned to use for another purchase), but we have to keep track of this debt. See, he's not even now, he actually has -10 dollars. So then he gets $90 back for the horse. But since he had -$10, the net effect of getting that $90 is to make him have 80 dollars (you might think about that as paying back the $10 he owed). So since he started out with $60, and he has $80 at the end, the overall effect was to gain $20.

I thought I understood it. Also got the right result.

(simply by regarding them separately and assuming that the man does indeed have enough money / doesn't need to borrow etc...)

But this explanation...?

mine would be something like this:

70 - 60 = 10.- (put it in the "profit wallet" or whatever)

90 - 80 = 10.- (put that in the same wallet)

The content of the wallet will be 20.- (unless there was money in there before we started the horse trading scenario...)

ohfortuna Cuisenaire are all the rage now! In fact, we have been using them for the last two weeks in year 2 while we work on part + part = whole and the inverse. The children love them!

Buys a horse for 60 means a minus 60 which = -60
Sells a horse for 70 means a plus 70 which = +70
Buys a horse for 80 means a minus 80 which = -80
Sells a horse for 90 means a plus 90 which =+90

Which = +20

It was only when FinancialDad (think that's their name) from earlier in the thread broke it down that it made sense!

If each purchase/sale is viewed as a different transaction e.g. the first time he buys and sells a horse and you work out the profit. Then the second time he buys and sells a cow and you work out that profit then it makes total sense!

Until then I had the profit as $10

I also got 20 profit

What about the 60$ he’s down to start with!?

Agree it's $20

What about the 60$ he’s down to start with!?

Yes, he recovered that, plus a profit of $20.

Hurray!
It took me a little while, but after writing all the figures down I see now and I understand the point where I went wrong.

I made the assumption that the $10 made from the first sale was needed to go towards the second purchase and that this was lost/cancelled out because of the increase in price.

Part of my brain still thinks this but the numbers don't lie, seems it is 20

Yay @Bookaboo! I think how easy it is depends on how your brain initially sees it. If you see that 10 in the middle as a loss, it's hard to unsee it, if that makes sense!

Made a tenner.

Not really a real world question is it. No mention of underlying interest rate (i.e. cost of money in this world), nor of what else the man could have been doing with his time, instead of selling horses for $10.

Insufficient data = no definitive answer.

Do I win ?

It has nothing to do with transactions. It is a simple minus and addition sum. Starting with a minus.

Just watching school of rock.

Awww I was a "10" person until I wrote this out...

He only has 60 in the whole world...
Buys horse for 60.
Sells horse for 70 = 10 profit
Simple.

He wants to buy horse for 80.
So has to borrow 10 from a friend as he only has 70 in his pocket.
Fine.

Then Sells it for 90.
But has to give 10 to his friend who he borrowed the 10 off for the 2nd transaction = so 80 is left for him

So he has 80 in his pocket and started with 60, so yes it is 20 profit.

Another way to look at it (for the "10" people) is that the goods have gone from 60 to 90 so a 30 increase and he only had to borrow 10 to make that happen = so he ends up with 20 for himself

No mention of underlying interest rate (i.e. cost of money in this world)
If you have money, and spend it, there is no interest rate -you're making up complications. By that measure, there is no answer ever, to any question about how much something costs (because someone somewhere may have taken a loan which needs factoring in).

I am awful at math and can work this out.

Haven't got a bloody clue. My Mathes is as bad as my Russian.

If it was so straightforward it wouldn't have warranted a thread Some people read something and get it first time, some don't.

I can’t understand how anyone gets an answer other that 20