Calculate, give the answer as a fraction.

a) [[$ 3^{-1} + 4^{-1} $]] =
b) [[$ 2^{-1} - 2^{-2} $]] =
c) [[$ 5^{-1} \cdot 3^{-2} $]] =
d) [[$ 3^{-1} : 2^{-2} $]] =