A hexahedron is the name for a 6 sided 3D shape.
the name of a 20 sided 3d shape is called a icosahedron
There is not a definite name for a 3D figure with 18 sides.It is possible that it is something else - but the information to test this is not provided.
Octadecadron
HEXADECAHEDRON is the correct name for a 16 sided 3D object
Tetrahedron
dodecahedron
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Dodecahedron
Octahedron
Pentacontakaihedron.
A 12 sided 3D shape is a dodecahedron.