“There were some people on a train. 19 people get off the train at the first stop. 17 people get on the train. Now there are 63 people on the train. How many people were on the train to begin with?”
The simple answer only accounts for how many got on and off the train, but it’s also possible that people were born or perished on the train along the way.
How many people jumped off before the first stop?
To begin with there was 1.