Mathematics: Interesting calculation using addition and multiplication

Yesterday I have discovered following calculation during playing with calculator. It is very interesting.....

1 x 8 + 1 = 9
12 x 8 + 2 = 98
123 x 8 + 3 = 987
1234 x 8 + 4 = 9876
12345 x 8 + 5 = 98765
123456 x 8 + 6 = 987654
1234567 x 8 + 7 = 9876543
12345678 x 8 + 8 = 98765432
123456789 x 8 + 9 = 987654321

100 - 1= 77.It's a correct mathematical subtraction!!!

100-1 =77. Are you confused with my mathematics? Let's explain......

The numbering system we use in daily life is called decimal number system where we have 10 digits from 0 to 9. And another type of numbering system is Octal Numbering System. In octal, we have only 8 digits starting from 0 to 7. So in octal numbering system, we will count this way 0,1,2,3,4,5,6,7,10,11,12,13,14,1,5,16,17,20,21......75,77,100,101.....107,110,101......and so. In octal, 7+1= 10. So 100-1=77 in octal number system. So I hope, your confusing is clear now.