Question on computer architecture?

Question

there is a question in my textbook which i still feel a bit confused on -

What is the maximum decimal number you can represent in 4 bits? 16 bits? N bits?

(N is a variable I'm assuming)

Answer 1

Figure out the number for any number of bits where all bits = 1.  See if you can detect the pattern.

Answer 2

15, 255, 2^N -1

(2 possibilitys (0 or 1), N Bits, And it can not be all 0 values so minus 1)