Given 2*4 decoder
1 decoder -------- 4 lines
l line ------- 1/4 Decoder
16 lines ------ 16/4 = 4 Decoder
4 lines -------- 4/4= 1 Decoder
4 lines ---- 4/8 -== 1 Decoder
The construction would be like :-
∴ total 4 + 1 = 5 decoder
As in general, let we want to $m$ x $M \; decoder\;with \;the \;help\; of \; $ $n$ x $N$ :-
No.of levels = $⌈\frac{m}{n}⌉$ = k
Total Decoders required = $\frac{M}{N}+\frac{M}{N^2}+\frac{M}{N^3}+\frac{M}{N^4}+........+\frac{M}{N^k}$