Cleo builds a pyramid with identical metal spheres.
Its square base is a 4 × 4 array of spheres, as shown in the diagram. The upper layers are a 3 × 3 array of spheres, a 2 × 2 array of spheres and a single sphere at the top. At each point of contact between two spheres, a blob of glue is placed.
How many blobs of glue will Cleo place?
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