This is a classic that I’m sure most maths teachers will know. I’m just hoping my Year 9 class won’t have seen it when I try it on them next week as part of a probability lesson.

In looking for some images I came across an on-line survey that Transum had done following an enquiry from a TV show. The results were interesting and worthy of a few minutes of discussion in class once the “trick” itself has been revealed.

If you haven’t seen this before, you essentially ask the following questions (respondents keep the answers in their heads):

The first part uses the fact that the sum of digits of multiples of nine is always 9, so that you always end up with a 4 which then turns into D. Most people think of Denmark and then Elephant, but not everyone does as demonstrated by the results of Transum’s online survey.

This is a nice intro to Tree Diagrams but also gives some scope for discussion of these figures, i.e.

- What’s the overall % of people that this trick works on?
- 68% seems low – would it be higher if it was a UK only audience? (rather than a global Internet audience)
- What were the percentages in our class?
- What percentage of people does it need to work on to have the desired effect in the room?

You could even follow this up with a homework to survey 10 people. Just don’t call it the “Elephant in Denmark trick” as my son did. Oh dear…