w children were riding on the bus. At the bus stop x children got off the bus while some more got on the bus. Then there were y children altogether on the bus. How many more children got off the bus than those that got on?
There were w children on the bus to start with. 

x children got off the bus, 

while some more got on the bus. 

Let's call the number of children who got on the bus z. 

Then we have the following system of equations: 

w - x = y 
x + z = y 

From the first equation we have: 

z = w - y 

Substituting this value of z in the second equation we get: 

x + w - y = y 

or 

x - y = w - y 

or 

x - y = y - w 

or 

x = w 

Thus, the number of children who got on the bus was equal to the number of children who got off the bus.