Answer: [120 ball pyramid] + [560 ball pyramid] = [680 ball pyramid]
def tetrahedral_number(n) return n*(n+1)*(n+2)/6 end #Build an array of tetrahedral numbers t_nums = [] for i in (1..100) t_nums << tetrahedral_number(i) end print "Tetrahedral Cannonball Stack Combinations:\n\n" str = "ball pyramid" #Find sums of tetrahedral numbers that are in the tetrahedral number set for i in (0..99) for j in (i..99) if t_nums.include?(t_nums[i] + t_nums[j]) total = t_nums[i] + t_nums[j] print "[#{t_nums[i]} #{str}] + ", "[#{t_nums[j]} #{str}] = ", "[#{total} #{str}] \n\n" end end end