What must be added to 7.68 to make 10?

You can figure this out by doing the opposite. Since you need to know "What added to 7.68 makes 10", let's reverse the question and say "7.68 subtracted from 10 makes what?"

Now we just do the math.

10 - 7.68 = 2.32

Well now we have our answer.

"2.32 added to 7.68 makes 10"