scramble prime challenge

[Paul Dale]

Off the top of my head, any number with a 5 or an even digit isn't allowed. The numbers can only be composed of 1, 3, 7 and 9.

By inspection there are five two digit possibilities.

Three digits has more cases. Divisibility by three rules out a single repeated digit, 117, 177 and all combinations that only contain 3 or 9. This brings it down to a more manageable number than 64 for manual checking. I've found three three digit possibilities.

Four, I'll need my calculator for.