You need to find four starting numbers to place at the bottom so that the top number will be 15.
Try to find as many sets of four starting numbers as you can.
Think about good ways to do this. Maybe use a system!
What little change can you make to the rules to explore again?
What new ideas can you think of to make this different? Explore those!
You may like to take a look at the follow-on activity Build it up More.