Suppose that a pair of 7 sided dice is tossed. what is the expected value of the sum?

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.

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Step-by-step explanation:

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