Good answers
Both riddles attempt to trick the reader into committing an accounting error.
Here's an actual example (From Facebook) of how the "I bought a cow" riddle does that:
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First,
the riddle says I bought a cow for $800. Thereafter I sold it for
$1000. So I make $1000-$800 = $200.
Now the 3rd statement says I bought it again for $1100. Here if
you see, I have bought the same cow again for $1100. So I had a loss
of $100 as previously the cow was sold for $1000 and bought again at
$1100.
Next statement of the riddle says I sold it again for $1300. So
now I have made $1300-$1100=$200 in this transaction.
So I made $200 – $100 + $200 = $300.
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The problem with the explanation above is that the $1100 figure has been subtracted twice, which in this case, results in a $100 error.
On an actual balance sheet, you would have Sales of $2300 and Expenses of $1900, giving a bottom line of +$400
The misdirection in the second riddle is entirely in the last line, where the manager adds the $2 to the $27 to arrive at a completely meaningless figure.
$30 was collected, of which $3 was given as change and $2 was kept as a tip leaving $25 as the actual cost of the meal. The order in which the $3 and $2 are subtracted doesn't matter as long as they're both subtracted.
The point here is that even the simplest of things can be obfuscated. Math cuts through bullshit like a machete, but misdirection takes many other forms.