cal black and white pattern. 50 2 3 2 3 2 2 2 3 2 2 3 3 3 3 3 3

1/21 2/21 3/21 4/21 5/21 6/21 7/21 8/21 9/21 10/21 11/21 12/21 13/21 14/21 15/21 16/21 17/21 18/21 19/21 20/21 21/21