1.  Think of a two-digit number (in the interval [10,99]) (for example 35).
2.  Add the two digits in this number (3+5 = 8).
3.  Subtract the result in 2 from the number in 1 (35-8 = 27).
4.  Find the result from 3 in the figure to the left,
    and notice the symbol to the right of this number.
5.  Click on the blue circular disk below this text. 
    The symbol you found in 4 will now be shown in the circular disk.