To get from 15 to 12, you subtract 3, and if you do you get 15-3 = 12. To get from 12 to 9, you subtract 3, and if you do you get 12-3 = 9. To get from 9 to 6, you subtract 3, and if you do you get 9-3 = 6. To get from 6 to __, you subtract 3, and if you do you get 6-3 = __. To get from __ to __, you subtract 3, and if you do you get __-3 = __. I'll bet you can now fill in those blanks. If you can't then ask me to help you in the thank-you note form below and I'll get back to you by email. Don't worry, I do this for fun and never charge a penny. Edwin