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 riding on the bus. 

Then, at the bus stop, some children got off the bus. 

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

So, if we start with 43 children, and then some children get off the bus, the final number of children on the bus will be less than 43. 

Now, we are told that there were 21 children left on the bus. 

This means that, if we start with 43 children, and then some children get off the bus, the final number of children on the bus will be 21. 

Therefore, the number of children who got off the bus at the bus stop must be 43 - 21, which is 22.