Fred traveled to New York City to meet with new clients. He took a cab from the airport to downtown New York. It took nearly an hour to get to his destination. His cab fare came to $67.00. How much should Fred tip the cab driver?

Some professions that provide you with personalized service don't pay well and so they expect a tip - waiters and waitresses, cab drivers, tour guides, hotel bell hops (yes they do still exist in many places), hotel maids, valet attendants (they park your car for you), movers and barbers and hair dressers.  
Often, tips are based on percentages.