Question has no solution. There must be an error.
6/n=18/n would require 6=18, which is never true.
Maybe you meant: 6/n+18/n=n/20?
24/n=n/20; 480=n^2; n=sqrt(480)=4sqrt(30)=21.91.
Or, perhaps 6/n=18/n-n/20?
120=360-n^2; n^2=360-120=240; n=sqrt(240)=4sqrt(15)=15.49.
Or 16/n ÷ 18/n=n/20?
16/18=n/20; 8/9=n/20; 9n=160; n=160/9=17.78.
Or 16/n=18/n ÷ n/20? 16/n=18/n * 20/n;
16/n=360/n^2; 16=360/n; n=360/16=45/2=22.50. (All answers rounded to 2 dec places.)