In a pyramid, with 4 circles at the bottom we could put 4 single digit numbers (less than 10 and 0 not allowed). The pairs of numbers were then added together to make the next row of numbers. And so on. The number at the top of the pyramid must be 15.
This activity developed our trial and error skills and resilience. We also had to really think ahead. We tried starting from the bottom and working up, then started at the top and worked down! What might be the problem with this?: