Some children were riding on the bus. At the bus stop w more children got on the bus. Then there were x children altogether on the bus. How many children were riding on the bus before the bus stop?
First, we need to find out how many children were riding on the bus before the bus stop. We know that at the bus stop, w more children got on the bus. This means that there were w + x children on the bus altogether. We also know that there were x children on the bus altogether. This means that there were w + x - x, or w, children on the bus before the bus stop. 

Therefore, the answer is: 

w