Using Bertrand's postulate which states:
For every integer $n \geq 1$ there is a prime number p such that $n<p\leq 2n$
Prove that there exists infinitely many primes whose decimal expansion starts with $1$.
I'm guessing I need to use something equal to $10$ in this as it is a decimal expansion, but I'm not seeing where to start?
Any guidance would be great, thank you