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%I #6 Sep 25 2019 11:00:23
%S 6,15,20,30,45,60,70,120,150,195,270,420,870
%N Strictly positive numbers n such that 30*n/(30+n) are integers.
%C The 30th row of A127730.
%C The ansatz 30*n/(30+n)=j (any integer j) yields n=30*j/(30-j) which demonstrates that the sequence is finite if n>=0. [R. J. Mathar, Jul 13 2009]
%t f[a_,b_]:=(a*b)/(a+b); a=30;lst={};Do[If[f[a,n]==IntegerPart[f[a,n]],AppendTo[lst,n]],{n,9!}];lst
%t Select[Range[1000],IntegerQ[(30#)/(30+#)]&] (* _Harvey P. Dale_, Sep 25 2019 *)
%Y Cf. A162688, A162689, A162690, A162691, A162692
%Y Cf. A127730. [From _Franklin T. Adams-Watters_, Aug 07 2009]
%K nonn,fini,full
%O 1,1
%A _Vladimir Joseph Stephan Orlovsky_, Jul 10 2009
%E Edited by _R. J. Mathar_, Jul 13 2009
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