с+4/6=3с-2/7
42с+28=126с-12
42с-126с=-12-28
-84с=-40
с= 10/21
====================
10/21+4/6=3*10/21-2/7
12/3=4 кг в 2 ящике
3,2845*643,4-328,45*6,434=
328,45*643,4/100-328,45*6,434=
328,45*6,434-328,45*6,434=0