數學的序列問題

x*y = 10a + b
= a + b + 9a ------- (1)

a+b = x ------ (2)

so x*y = x + 9a
x*(y-1) = 9a

When x = 9,
y=1, a=0
y=2, a=1
.......
y=9, a =8

When x =6, 2(y-1)=3a
y=4, a=2
y=7, a=4

When x=3, y-1=3a
y=4, a=1
y=7, a=2