This Math Problem For 15-Year-Olds Stumped Many Adults

According to a survey, a significant number of adults could not solve this math problem that was intended for 15-year-old students. Here's the problem: There were some people on a train. 19 people get off the train at the first stop, 17 people get on. Now there are 63 people on the train. How many people were on the train to begin with?