Susan solved 200-91 and decided o add her answer to 91 to check her work. explain why this strategy works

Think of an equation:

200 - 91 = 109

You can do anything to this equation, as long as you do the same thing to both sides. So, if you add 91 to both sides, you will get:

200 = 200

Since this holds true, Susan's strategy to add her answer to 91 makes sense.