Put nine horses into four rectangular pens so that each pen contains an odd number of horses.
Is there more than one answer? If yes, how many?
Hint: Use nine pennies and place them in drawings of possible pens. What do you discover?
Solution Commentary: Think...the only odd possibilities are 1, 3, 5, 7, and 9. Experimentation should lead you to conclude that the problem seems impossible...until you make your mind flexible. That is, focus on the assumptions that you might be making.
As a final hint (without giving away the solution), think about how one set can be a subset of another larger set...plus that 3=1+2, even when the numbers refer to horses.