∵ Function image passing point (50,250), (200, 100),

∴50k+b=250200k+b= 100，

The solution is k =? 1b=300，

∴y=-x+300；

(2) From the point of view of the title, 3000? x+300-3000x= 15，

After naming and sorting, x2+ 100x-60000=0.

The solution is x 1=200, x2=-300 (truncation),

y=-200+300= 100，

A: There are 200 pencil cases, and 100 pencil cases were purchased.