(1)由题意得:
y=20+2(40-x)
=-2x+100.
则y与x的函数关系式为y=-2x+100;
(2)根据题意得:
w=(x-18)y
=(x-18)(-2x+100)
=-2x2+136x-1800,
则w与x的函数关系式为w=-2x2+136x-1800;
(3)令w=480,得480=-2x2+136x-1800,
整理得x2-68x+1140=0,
解得x1=30,x2=38,
当x=30时,y=-2×30+100=40(万件),
当x=38时,y=-2×38+100=24(万件),
答:此时的销售单价是30元,月销售量是40万件或销售单价是38元,月销售量是24万件.