Percentage is in very common usage in our society, especially in financial issues. Stores will often advertise a 50% off sale. Banks will list loan interest rates, and savings account interest rates as a percentage of the balance per year. Percentage is also often used in astronomy when referring to a fraction of something. For example, the Earth has had multi-cellular life for 16% of its existence, or approximately 50% of all solar systems have more than one star. To be a successful astronomy student or an informed consumer one must know what these percentages actually mean. Fortunately, gaining this skill is one of the more straightforward math skills to acquire.