a ppt is here
the second resource (not numbered, between (1) and (2)) is to detect those students who fairly routinely prefer to subtract the smaller digit away from the larger in a column; hoping that they might appreciate the error of their ways...
sheets (1) and (2) give particular multiples of 9 and 99 and these are patterns that can be explored further
sheets (3) and (4) involve all the digits, 1 to 9 and then 0 to 9
students can consider how to obtain other examples by swapping some of the digits