Question 880826
Let A be 100, for the sake of easiness.
10% of 100 is 10
5% of 100 is 5.  5% "again" is 5% of 95, or 4.75.  Total 9.75
.
You've probably seen this in department stores.  "Extra 50% off already reduced merchandise."  Wow, a beautiful dress for $100 marked down to $50.  Shouldn't this be free?  No, it is 50% off the already discounted price of $50, or 75% off, not 100%.  Still a good deal :-).