One-year procurement cost = 250 times
Average inventory = (30000/x+0)/2 =15000/x.
Average inventory amount = 4 *15000/x = 60000/x.
Annual inventory cost = 60000/x *15% = 9000/x.
Annual total expense Y= annual purchasing expense+annual inventory storage expense = 250x+9000/x.
Namely: y = 250x+9000/x >; =2* radical sign (250X*9000/X)=3000.
When 250X=9000/X, that is, X=6, the equal sign holds.
Purchase 6 times a year, each time 30000/6 = 5000 kg is appropriate. The purchase interval is 60 days, and the total annual cost is 3000 yuan.