This is an exercise from old exam on formal languages that I don't know how to solve:

Let $p \ge 5$ be a prime number and $L_p$ be a language of words over $\{0,1\}$ that read in binary from right (i.e. from least significant bit) give a number that gives remainder modulo $p$ from the set $\{1,2, \ldots, \frac{p-1}{2}\}$.

How to show that:
> Every DFA recognizing $L_p$ has at least $2p$ states.

?

One fact that I know of and is somehow related (has DFA and primes in the statement) is:
> Any DFA recognizing language $\{0^n : n \text{ is not divisible by } p\}$ has at least $p$ states.

This can be seen by observing that the language is infinite, hence any DFA must have a reachable cycle, from which some accepting state is reachable. And if that cycle had less than $p$ states, then because any number smaller than $p$ is coprime with $p$, we could loop sufficiently many times in that cycle and arrive at the aforementioned accepted state with a word $0^{kp}$ for some natural $k$ - a contradiction.

Maybe it's possible to use this fact, or alter this proof somehow to make it fit for the theorem with $L_p$?