w children were riding on the bus. At the bus stop some children got off the bus. Then there were x children left on the bus. How many children got off the bus at the bus stop?
There were w children on the bus. 

Some children got off the bus. 

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

So now there are x children on the bus, which means that x is less than w. 

That means that the number of children who got off the bus is w - x.