What Is N?

"If 10 - N = 5, what is N?" I asked Supriya. She thought for a couple of seconds, got her mind around the question, a light came on in her eyes and she answered, "5!" We worked with this for a while, I used M instead of N and it looks like she gets the idea of using a place-holder for the unknown entity (I avoided using X because it doubles as the multiplication sign as well.)

Then she gave me a problem, "6 - M = 5". I drew 6 dots and said, "If I take away 1 dot, I get 5 dots, which is what you want - so M is 1." This satisfied her.