Practice Percents in This Online Game:

Another game, Penguin Waiter Percent Game, is Funbrain's method of sharpening students' skills with percentages.  The penguin arrives with the bill for the meal, for which students then calculate the tip.  They also have a range of difficulty with the problems, getting as complex as working backwards to calculate the total bill amount based on the tip left.

                                                                                                                  from funbrain.com