Question

How many two input NAND gates are required to perform the action of a two input OR gate??

Answer 1

The answer is three.

Three two input NAND gates are required to perform the action of a two input OR gate. The single outputs from each of two two input NAND gates form the two inputs of the third NAND gate. This combination performs the same function of a two input OR gate.

Answer 2

We can use 3 NAND gates to perform the action of an OR gate.

We first use 2 NAND gates, and their outputs are connected to the third NAND gate, which then produces the same result as an OR gate.

If X is the initial input in the first NAND gate, we get
(X.X)`
= X`

If Y is the initial input in the second NAND gate, we get
(Y.Y)`
= Y`

When we connect these two outputs to the input of the third NAND gate, we get
(X` . Y`)`
= (X`)` + (Y`)`
= X + Y
which is an OR gate.