w children were riding on the bus. At the bus stop x children got on the bus while some got off the bus. Then there were y children altogether on the bus. How many more children got on the bus than those that got off?
There were w children on the bus at the beginning. 
x children got on the bus, which means that y = w + x children were on the bus in total. 
Now we need to know how many children got off the bus. 