Why do this problem?
Use this activity
to introduce the youngsters to a simple problem solving activity and to allow them to develop very individual approaches to gaining solutions.
Try some practical approches with blocks and get a discussion going to reinforce what is and what is not allowed.
How many across have you gone?
Can you find a way of doing this so that if you had the time you'd know you had found all the ways?
Explore variations like 3 across, 4 up etc.
Using 3D blocks all through the the work will help all children access this challenge.