The way the solution is written, it seems to work like this: The envelopes either contain $1000 or $2000. However, they are not marked so you cannot know how much is in the envelope. You friend hands you one (E1) and then offers to switch the second (E2). If E1 has $2000 in it, then E2 has half that, $1000. If E1 has $1000 in it, then E2 has double that, $2000. But, since the envelopes are unmarked, you cannot tell which is which using logic, so it does not matter which one you take. And quite frankly, if you have a friend who has $3000 to fiddle around with in this way, just kick him in the nuts and take both envelopes. His money would be better spent by you, anyway, apparently.

Now, here's another one for you:

Three men get a hotel room to play poker all night. The day manager tells them the room is $30, so each man pays with a $10. $10 + $10 +$10 = $30. Later, the night manager comes in and goes over the books and finds that the men were overcharged. The room cost $25, not $30. So he calls a bellboy over and pulls five $1 bills out of the till and sends them up to the men with his apologies. $30 - $5 = $25. In the elevator, the bell boy realises that the men may fight over who will get shorted the dollar, so he "selflessly" puts two of the $1 bills in his wallet and returns the three $1 bills to the men. So now the men paid $10 - $1 = $9, times three men, $9 x 3 = $27, Plus the two $1 bills in the bell boy's wallet. $27 + $2 = $29. Where's the last dollar?

Do not say taxes.