There are fifteen apples on the tree.six are on the ground.which number sentence shows how to find the total number of apples?

how about:

add the number of apples in the tree to those on the ground

??

None, apparently. =(

To find the total number of apples, you need to add the number of apples on the tree to the number of apples on the ground. In this case, there are fifteen apples on the tree and six on the ground. Thus, the number sentence that shows how to find the total number of apples is:

15 + 6 = ?