Or run the times table on 9 and the answers from 1x9 to 10x9 are:

09...18...27...36...45...54...63...72...81...90

Note the numbers from 54 are inverts of the first series AND that the two digit answers all add to nine. I used that plus two of the above when I taught Math in a Vo-Tech where all the kids pretty much hated math and had done poorly before. All of a sudden they got curious and curiosity is one of the best things you can get to work with them.

Spaw