One of the most common ways that lawyers use math are percentages. For example, they many need to calculate what the odds are they'll win a trial. Divorce lawyers especially use percentages when it comes to deciding what assets go to which spouse. Settlements are another way that lawyers use percentages to ensure that their client gets a fair compensation for the actions of others.
Geometric proofs are also a common math skill used by lawyers. Some may not even realize they're using geometric proofs. The lawyer takes all of the facts and then applies the laws that relate to her clients--keeping these facts in line--then uses deductive logic to figure how to fight her case in a successful way.
Lawyers who work in more complicated fields--such as tax fraud, or even copyright law for new inventions and discoveries--often use complex math in their jobs. Formulas are common, especially for those in copyright law. This is because many times, the lawyer has to at least partially understand the engineering aspect behind an invention that he's helping his client copyright. Tax fraud lawyers need mathematical formulas to help look through mountains of financial numbers and process them into a clear-cut case.