Lets say that 's' stands for 'sandwich'. It could be any noun you want that starts with the letter 's'.
9s represents having 9 sandwiches
5s means you have 5 sandwiches
and so on
Saying 9s-5s means you start with 9 sandwiches and take away 5 of them. So, 9s - 5s = (9-5)s = 4s. We're left with 4 sandwiches.
Then add on 2 more
9s - 5s + 2s = 4s + 2s = (4+2)s = 6s
We now have 6 sandwiches
Finally, take away 1 sandwich. The last "-s" in your given expression is the same as "-1s"
So,
9s - 5s + 2s - s = 9s - 5s + 2s - 1s
9s - 5s + 2s - s = 6s - 1s
9s - 5s + 2s - s = (6 - 1)s
9s - 5s + 2s - s = 5s
or you could write your work like this
9s - 5s + 2s - s = 9s - 5s + 2s - 1s
9s - 5s + 2s - s = (9 - 5 + 2 - 1)s
9s - 5s + 2s - s = 5s
Either way, the given expression simplifies to 5s. In the end, we have 5 sandwiches.
