Okay. Let's pretend that you have 9 toy cars. You love these toy cars, so your parent get's you 7 more toy cars. You're really excited now, and you're counting how many you have by putting them into a pile of 7 and a pile of 9. You do so by taking one of the 7 cars away from the 7 pile, and you add it to the 9 pile. Now you have a pile of 10 cars, and a pile of 6 cars. Now, you just add the 6 to the 10, and you have 16 cars. This strategy is pretty simple, and extremely helpful. :D Although, I've never heard of doing math like this before, so this was something I had to look up before I actually gave you an answer. If you need more information, just tell me and I'll give the information to you. Otherwise, if I don't, please remove my answer so no one else get's the wrong answer. :D