w children were riding on the bus. At the bus stop some children got off the bus. Then there were x children left on the bus. How many children got off the bus at the bus stop?
There were w children on the bus at the start. Then, some children got off the bus at the bus stop, so there are now x children left on the bus.

We can say that x = w - y, where y is the number of children who got off the bus.

We want to solve for y, so we can rearrange the equation to get:

y = w - x

So, the number of children who got off the bus at the bus stop is equal to the initial number of children on the bus, minus the number of children who are left on the bus.