Prove that if the digits of N, read from left to right, form a strictly increasing sequence, the sum of the digits of 9*N is always 9 (nine).

You are requested to formally prove, not to verify.

(In reply to Aha! (spoiler) by Steve Herman)

I, too, was looking forward to seeing someone prove this.  I had no such flash of insight. 

That was great!

Posted by Jer on 2015-09-30 17:32:16