Determine the efficiency of a shannon-fano binary code

Question

Assignment:

A quaternary memoryless source has alphabet Ax = {0, 1, 2, 3} and PMD

P_x(0) =1/3, P_x(1) = 1/6, P_x(2) = 1/2

and is coded symbol by symbol.

a) Find the efficiency of a binary code for it with fixed length words.

b) Find the efficiency of a Shannon-Fano binary code for it.

c) Find the efficiency of a Huffman binary code for it.

d) For what PMD would a binary variable length code with unit efficiency be possible?