Which one of the following is NOT a valid identity?

What is the minimum number of 2-input NOR gates required to implement a 4-variable function function expressed in sum-of-minterms form as

f = Σ(0, 2, 5, 7, 8, 10, 13, 15)?

Assume that all the inputs and their complements are available.

In 16-bit 2's complement representation, the decimal number -28 is :

Two numbers are chosen independently and uniformly at random from the set {1, 2, ...., 13}. The probability (rounded off to 3 decimal places) that their 4-bit (unsigned) binary representations have the same most significant bit is ______.