Q: How many 3 to 8 decoders with an enable input are needed to construct a 6 to 64 line decoder without using any other logic gate.

(A) 7 (B) 8 (C) 9 (D) 10

Ans given (B) 8

We need 8 3×8 decoders for 64 outputs. Now these 8 are having one extra enable input (which is separate fr0m actual 3 inputs). So to select any decoder fr0m these 8 we need another 3×8 decoder (say 9th decoder) output of which will be connected to each enable input of those 8 decoders.

Now for 9th one do we need any separate enable input ? If yes then out of actual 6 inputs which one will be connected with the enable input of 9th one? So the 9th one is the simple decoder which has no separate enable input.

Question asked how many 3×8 decoders with enable line. Here we need total 9, but among those 8 with enable inputs. I think intention of question is to ask this only. In that case answer 8 is correct.

Please let me know your view.