Consider a pyramid constructed of stacked spheres (e.g. a pyramid of oranges). The first layer of the pyramid has only one sphere. The second layer (just beneath the first) has 3 spheres. The third layer has 1 + 2 + 3 = 6 spheres and the $Nth$ layer has 1 + 2 + 3 + ... + N spheres.

Express the number of spheres in the $Xth$ layer as a function of X.

And the answer is...