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Q.
How many NAND gates are used to form AND gate?
Semiconductor Electronics: Materials Devices and Simple Circuits
Solution:
Two NAND gates arranged as shown, would give an AND gate.
Here output from first NAND gate is $\overline{A \cdot B}$
This, as input, is again fed to a NAND gate, giving
$\overline{\overline{\text { A.B. }} \cdot \overline{\text { A.B }}}=\overline{\overline{\text { A.B }}}+\overline{\overline{\text { A.B }}}= A.B$