I got 22 on my first try. The most helpful trick is to note that if the digits add up to a multiple of 3 it cannot be prime (because it is divisible be 3) with

I got 22 on my first try. The most helpful trick is to note that if the digits add up to a multiple of 3 it cannot be prime (because it is divisible be 3) with the exception of 3 itself.