An interesting problem to try. Discuss with your class first to see if they can come up with a logical strategy to attempt to solve it. (If they get stuck with their approaches, you may wish to suggest that they work backwards, and/or use materials)
NOTE: I do not usually supply answers, as I prefer teachers and students to work towards an answer that best suits them at the time. If teachers have a method and/or answer they can inadvertently lead students to the solution without them undertaking the investigation fully.