An appropriate tip for a waiter at a restaurant is at least 15% of the total bill. For a tab of $20, the tip would be:
Tip amount = 15% of $20
Tip amount = 0.15 x $20
Tip amount = $3
Therefore, the total amount of money that should be left, including tip, is:
$20 (tab) + $3 (tip) = $23.