Question 7
An $a\times b\times c$ rectangular block with $a\le b\le c\le20$ is made from unit cubes. When it is viewed from one vertex, such that the three faces intersecting at this vertex can be entirely seen, exactly 487 cubes are visible. What is the number of the unit cubes not visible? List down all possible answers.
For example, the diagram on the right is a $3\times4\times5$ rectangular block. When it is viewed from vertex $A$, 36 cubes are visible.
For example, the diagram on the right is a $3\times4\times5$ rectangular block. When it is viewed from vertex $A$, 36 cubes are visible.