IF such dice exist, and
IF each die is numbered from 1 to 7, and
IF each die is fair
then
expected value of each die is 7/2=3.5, and
by the rule of sum of random variables,
E[x]+E[y]=E[x+y],
then
expected value of the sum is 3.5+3.5 = 7.
Note: the platonic solids (convex regular polyhedrons) that can be used as fair dice are
tetrahedron (4 faces)
hexahedron (6 faces)
octahedron (8 faces)
dodecahedron (10 faces)
icosahedron (20 faces)
All of these dice exist in the market.
