Sekadar itung-itungan.

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