The set has $7$ elements.
Work Step by Step
A set has $127$ proper subsets. The formula for the number of proper subsets when the set has $n$ elements is: $2^n-1$. $2^n-1=127$ $2^n=128$ $2^n=2^7$ $n=7$ The set has $7$ elements.
You can help us out by revising, improving and updating this answer.Update this answer