设每吨材料的售价为x元(x≤260),则每吨的利润为(x-100)元,每吨售价降低了(260-x)元,月销售量增加了7.5(260-x)/10吨,实际月销售量为[45+7.5(260-x)/10]吨,根据题意,有：
y=(x-100)[45+7.5(260-x)/10]
=(x-100)(45+195-0.75x)
=(x-100)(240-0.75x)
=240x-0.75x²-24000+75x
=-0.75x²+315x-24000
所以
y= -0.75x²+315x-24000