|9 + 1 = 6|
Write 'IX' on a flip chart or white board and ask the group how the figure can be converted into a '6' with the addition of only one line.
If you want to make the puzzle more difficult, try telling this story:
A baker used to send the cakes he had not sold to his old mother. He always asked his apprentice to deliver the cakes on his way home. After a while, the baker started to suspect that the apprentice was eating some of the cakes, so he started to write the number of cakes on the box. One day the apprentice ate a third of the cakes and corrected the number on the box by adding only one line. How many cakes were in the box when the apprentice left the bakery?