(15,11) Hamming Code, too work out how to calculate check digits

[jackscholar]

Homework Statement

I need to figure out how to find out the amount of check digits in a (15,11) hamming code. I know the amount of check digits is four and the amount of information digits is 7 but I don't know how to figure out how to get the answer, I only have the answer.

Homework Equations

I was given the equation n=4+c
n=2^c - 1
4+c=2^c-5
c=2^c-5

The Attempt at a Solution

I do not understand how the equation above is possible, so i could not figure out how the result c=2^c-5 was achieved

 

[rcgldr]

Normally a (15,11) hamming code would mean 15 total bits, 11 of which are data.