Let the truth be told!
“Now if you do the math, together they paid $30, got $3 back meaning they only paid $27. The waitress kept $2. This totals $29. Where did the last dollar go?”
– Thats right, they PAID $27 ($25 for steaks, $2 for tip) and still have $3 of the original $30
They had $30. $25 went to steaks, $2 went for tip, thats the $27 they spent (note: the tip is already accounted for). They still have $3, $27 (spent) plus $3 (kept) equals $30.
The problem is that people don’t think of the tip as part of the money spent because of the sentence “The waitress kept $2” which triggers the assumption that it is not yet accounted for. Then they strengthen your assumption by agreeing with you “This totals $29”.
Here it is with subtraction (correctly):
Total amount-steaks-tip=money left over
$30 – $25 – 2 = 3
Its a play on words that I’ve tried to explain to people many a time, the original statement is falsely stated, deal with it 
What really happened: Together they paid $30, but their total expense was $27. So $3 was returned to them. This totals $30. No problem.
