5

It's a tricky one but I think the idea is as follows


2

Here is my explanation. He buys the goat for 60. He sells it for [sealed box of money who knows how much is in there.] Then he buys it again for [sealed box of money who knows how much is in there] plus ten. He has spent a total of 70. He sells it for 90 so his profit is 20. This doesn't rely on adding two separate profits and then wondering if one of them ...


1

The logic error here is that coming up with a profit of only 10 subtly double-counts the two middle steps. As others have pointed out, the basic math is $-60+70-80+90=20$. The faulty logic is 10 profit, 10 loss, and 10 profit summing to 10 profit overall. However, the first 10 profit comes from $-60+70$, the loss comes from $70-80$, and the second profit ...


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