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. 
y children were on the bus in total. 
z children got off the bus. 

y = w + x - z

x - z = y - w