[Note: Some have suggested the total overhead paid to SB is only 5% after cancellation. The chips cost SB .078BTC each - he charged .086BTC, but is only refunding .081BTC. Sure looks like 15% to me.]
10% of 0.078 = 0.0078 <-- SB's markup.
0.078 + 0.0078 = 0.0858 <-- Cost per chip from SB rounded up to 0.086.
95% of 0.086 = 0.0817 <-- Refund from SB as stated in the email and on forums.
100% - 95% = 5% <-- BTC lost in the course of doing business. Seriously. This happens. Check the thread for evidence of work being done.
May I suggest
https://www.khanacademy.org/ for your remedial maths