There is no missing dollar. The easiest way to see that there is, in fact, no missing dollar is to track the money as it is given. After getting a $30 bill, each person (P1, P2, and P3) gives $10 to the waiter. Now the waiter has $30. The waiter then gives $25 to the cashier (C) and still has $5 (25+5=30). The waiter (W)decides to keep $2 dollars and give P1,P2, and P3 each a dollar. So (C + W+ P1 +P2+P3) = ($25 + $2+ $1 +$1 +$1) = $30.
The issue is that the next paragraph insists that each woman paid $9 each but that isn’t true. Since the bill is $25 and P1,P2, and P3 paid together, they all actually paid for the $25 bill with $5 extra. There is no way for 3 people to split $25 dollars evenly, as if they all pay $8 each it would total $24, so one person essentially paid a dollar extra. If P1 paid the dollar extra, then P1 + P2 + P3 + Extra = $9 + $8 + $8 + $5 = $30. So all of the money is accounted for.
Rowan Counterintuitive 