Body: 43 children were riding on the bus. At the bus stop some children got off the bus. Then there were 21 children left on the bus.
Question: How many children got off the bus at the bus stop?
question_concat: 43 children were riding on the bus. At the bus stop some children got off the bus. Then there were 21 children left on the bus. How many children got off the bus at the bus stop?