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?
There were y children riding on the bus before the bus stop. 

There were x children altogether on the bus. 

At the bus stop, w more children got on the bus. 

Some children were riding on the bus. 

y + w = x

y = x - w