Here:
  Number of vertices (V) = 20
     Number of edges (E) = 30
Let the number of faces are F. Then using Euler's formula, we have
                  F + V = E + 2                                                      ...(1)
∴         Substituting the values of V and E in (1), we get
                   F + 20 = 30 + 2
              F + 20 = 32
                     F = 32 - 20
                     F = 12
   Thus, the required number of faces = 12.