8/13(3 3/4m-5/16n)-3/20(6 2/3m-4 4/9n)=
8/13*15m/4-8/13*5n/16-3/20*20m/3+3/20*40n/9= =2 4/13*m-5/26*n-m+2/3*n=1 4/13*m+37/78*n
(2/3) - (5/26) = 37/78 и это НЕ равно (1/6) !!