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 some children riding on the bus. 
At the bus stop, more children got on the bus. 
Then there were x children altogether on the bus. 

We want to know how many children were riding on the bus before the bus stop. 

We can set up an equation to solve this problem. 

Some + more = x

Some children were riding on the bus. (We don't know how many, so we'll call this number "some.")
At the bus stop, more children got on the bus. (Again, we don't know how many, so we'll call this number "more.")
Then there were x children altogether on the bus. (We know how many children were on the bus in the end, so x = the number of children on the bus in the end.)

Some + more = x

We don't know the values of "some" or "more," but we do know the value of "x." 

We can solve this equation for "some" by subtracting "more" from both sides and then subtracting "x" from both sides. 

Some + more = x
-more -more