When you ad the 2 numbers you start with the number farthest to the right. So you add 5 and 5 to get 10. You then carry the 1 and now you add 1+2+7. First just add the 7 and the 2 to get 9. Add the 1 after that and you get 10 again. Carry the 1 onto the 2 and now you have 2+1. 2+1= 3
