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. Let's call this number $x$. 
At the bus stop some children got off the bus. Let's call this number $y$. 
Then there were 21 children left on the bus. Let's call this number $z$. 
We know that $x = y + z$. 
We are looking for $y$. 
We know that $z = 21$. 
Therefore, $x - z = y$, $x - 21 = y$, $43 - 21 = y$, $22 = y$. 
22 children got off the bus at the bus stop.