What is the maximum number represents by 16 bit binary digits?

Answer 1

The answer is 65535.

Where the a^b means a raised to the b power:
1 = 10 - 1 = 2^1 - 1 = 2 - 1 = 1

11 = 100 - 1 = 2^2 - 1 = 4 - 1 = 3

111 = 1000 - 1 = 2^3 - 1 = 8 - 1 = 7

1111 = 10000 - 1 = 2^4 - 1 = 16 - 1 = 15

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1111 1111 1111 1111 = 1 0000 0000 0000 0000 - 1 = 2^16 - 1 = 65536 -1 = 65535