The composite digits are 4, 6, 8 and 9.
Clearly the number must have 2 or more digits so must end in 1, 3, 7, or 9.
Since 9 is the only composite, it must end in 9.
Checking 2 digit numbers with a composite first digit:
49 not prime
69 not prime
89 is PRIME

Posted by Jer
on 20150224 08:29:41 