Tip Calculation

An option to calculate the tip for you. For example, I type $20.00 for my meal and have an option to press add tip. It would bring up a slider/number pad that asks for the percent and just automatically calculates it and adds that to the cost.
It would also be nice to be able to see the separation between the meal cost from the tip when I click on the transaction, but not make them completely separate transactions. Thanks!