Respuesta :
"None" is a your clue word... if "none" means zero, then there is still "6 birds" because 0 left.
In a equation it will look like this: 6 - 0 = 6
In a equation it will look like this: 6 - 0 = 6
there are are still 6 birds left in the tree, because 0 of them have left the tree. so 6-0=6