设甲一件x元,乙一件y元,则丙一件(315-3x-7y)元,甲乙丙各1件共需x+y+(315-3x-7y)=315-2x-6y元.4x+10y+(315-3x-7y)=420,x+3y=105,所以,315-2x-6y=315-2(x+3y)=315-2*105=105.甲乙丙各1件共需105元.设买甲1件,乙3件要x...