Answer should be 6.
We are using 1x2 decoder . It means one input line will be decoded into two output lines.Now since we need 8 output lines (for 3x8 decoder ) so we can solve it like this :
2 lines can be generated by 1 line .
1 ,, ,, ,, 1\2 lines.
8 ,, ,, ,, 8\2 lines. = 4 decoder
Now 4 lines needed as output so
4 lines can be generated by 4/2= 2 decoder
Now for these 2 lines we can use
2 lines can be generated by using NOT gate.
The implementation will look like :-
So total decoder needed will be 4+2 = 6