The text Mathematics for the Trades (1996) discusses the "Sexy Six," a title which caught my attention.
Now, the "Sexy Six" are the six multiplication facts missed the most by students. Any idea what the six facts are? Make a guess before you continue reading...(I know, a presumption on my part!)
The text's authors included no research results to validate their claim (i.e. neither the most-missed aspect nor their sexiness).
The six facts are:
9 x 8 = 72
9 x 7 = 63
9 x 6 = 54
8 x 7 = 56
8 x 6 = 48
7 x 6 = 42
I always knew 8 x 7 = ? was a difficult fact...and have told students to think: 56 = 7x8. Great conceptual teaching on my part!
And, it is important to note that after their discussion of the "Sexy Six," the authors write this disclaimer: "There is nothing very sexy about them, but we did get you attention, didn't we?"
Yes, you did...and I passed the gimmick on...
Source: R. Carman and H. Saunders' Mathematics for the Trades (1996), p. 29