i'm getting back into first year university math after a huge amount of time. then hopefully, get into higher level math.
i find two things as i'm going through (they somewhat applies to the hard Q here)..
1) sometimes i just know definitions..... in fact, i lost hypothetical final jeopardy (i.e. i was watching on TV because i didn't know if 1 or 2 are prime. in fact, i might not have even thought of 2 as being prime - all primes are odd, i thought wrongly).............. in the question i posted, i think it means no remainder when divideed by 3
2) same as #1 but notation......... so easy to get scared by notation that turns out to be largely trivial (not all of the notations are easy, but many are)
as per the question, i think you make #'s out of 0,1,2,4,5,7,8... and then each of these is either a multiple of 3 plus/minus 1. so you like a card count in 21, you can track the divisibility of the 5 digit number (i think i have that right.. don't all numbers divisible by 3 make a dig sum divisible by 3.. haven't thought that through. just memory)
thx advance