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.
