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 to begin with. 

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

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

After the children got off the bus, there were x children left on the bus. 

That means that x is less than w. 

To find out how many children got off the bus, we need to find out the difference between w and x. 

w - x = the number of children who got off the bus