Question

What is the value of 1-18+25 if you subtract then add?

If you add then subtract?
Why is order of operations necessary?

Answer 1

If this case You can follow any order. If in the expression appear multiplication or division You must take this into consideration earier thatn subtract/add.

1-18+25=8

You can do it as You want
-17+25=8 or 1+7=8

Answer 2

The answer to that is always 8, it doesnt matter if you do 1-18 then add 25 or do -18+25 then add one.