autofill numbers

hi,

I am totally new to vba, and need help with the code.

I have 20 numbers need to automatically fill in column a and column b in the way of:
A1: 1 B1: 1
A2: 1 B2: 2
A3: 1 B3: 3
............
A20:2 B20: 20
A21:2 B21: 1
A22:2 B22: 2
A23:2 B23:3
A? :2 B? : 20
...........

till
A? :20 B?:1
A? :20 B?:2
A? :20 B?:3
A? :20 B?:20

for the extension of this, I also need to fill in column C, so the idea will be as below:
A1: 1 B1: 2 C1:1
A2: 1 B2: 2 C2:2
A3: 1 B3: 2 C3:3
.........................
A20:1 B20:2 C20:20
A21:1 B21:3 C21: 1
A22:1 B22:3 C21: 2
A23:1 B23:3 C21: 3
--------------------
A? :1 B?: 3 C?: 20
.........................
till
.........................
A? : 20 B?: 20 C?:18
A? : 20 B?: 20 C?:19
A? : 20 B?: 20 C?:20

Please help, thank you!

The logic is pretty simple.

Column A stays the same value up to multiples of 20 rows...
=INT(ROW()/20)+1

Column B increases until a row multiple of 20 is found, then restarts
use a seed of 1 on B1
B2, copied down...
=IF(A2=A1,B1+1,1)

However, it looks like you keep increasing past the 20th row in column B, but not column A?

Perhaps you need this in column A?
=INT(ROW()/21)+1

I did, in post #3

Column A stays the same value up to multiples of 20 rows...
=INT(ROW()/20)+1

Column B increases until a row multiple of 20 is found, then restarts
use a seed of 1 on B1
B2, copied down...
=IF(A2=A1,B1+1,1)

looks like a few typos in the example as op says 20 numbers

a1
=CEILING(ROWS($1:1)/20,1)
b1
=MOD(ROWS($1:1)-1,20)+1
for the second bit including col c i cant see whats required