A DMS has an alphabet of five letters x i $,$ i $=1,2,...,5,$ each occurring with probability $1/5$

Evaluate the efficiency of a fixed$-$length binary code in which

$1.$ Each letter is encoded separately into a binary sequence.

$2.$ Two letters at a time are encoded into a binary sequence.

$3.$ Three letters at a time are encoded into a binary sequence.