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?
There were 43 children on the bus. 

Some children got off the bus. 

That means that the number of children on the bus decreased. 

So 21 is less than 43. 

That means that 22 children got off the bus.