(2) ;
(3)
In order to obtain the maximum monthly profit, the material price should be set at 2 10 yuan per ton;
(4) I think Xiao Jing is wrong;
Reason: When the monthly profit is maximum, X is 2 10 yuan.
For monthly sales,
When x is 160 yuan, the monthly sales volume w is the largest.
When x is 2 10 yuan, the monthly sales volume w is not the largest.
Xiaojing is wrong.