So basically you can add it with column addition or just simply break it down
Think about it lets break down 12 i got 10 and 2
then you can add it more easily
you could do 5019+10=519
then add 2 to get 521
Or you could do this
509
12 +
______
521
so basically toy add 9+2 to get 11
then you carry the 1
then you do 1+1=2
then the 5 is left
so finally you get left with 512
You always work from left to right
