riddles

Answer

Dear Annick,

you can see how this ‘mind reader’ works as follows: the number you are asked to calculate is always divisible by nine. Indeed, suppose the number in your mind is given by:
A + 10 x B
Where A and B are two digits (so 0,1,2,3,4,5,6,7,8 or 9). Then the number you are asked to calculate is:
A + 10 x B – A – B = 10 x B – B = 9 x B
So this is always divisible by nine, and it can never be 90.

The computer uses this by placing the same sign at 9, 18, 27, 36, 45, 54, 63, 72 and 81 in the table. That way the computer knows in advance which sign you have landed on… You can easily test this by looking at the symbol in the table that corresponds to the nine (or to the 18, 27, 36, 45, 54 , 63, 72 or 81). You will see that it is always that symbol that the computer displays after clicking on the gray area.