Answer:
500
Step-by-step explanation:
Assume camel stays at the market after complete delivery.
Idea is to drop off some bananas between the plantation and the market. Refrigeration is required to avoid bananas going bad!
Status (planatation, 250 mile point, 500 mile point, market)
First trip with 1000 bananas, stop at 250 miles, drop off 500 bananas, and come back to plantation. (status: 2000, 500, 0, 0) camel ate 500 bananas.
Second trip with 1000 bananas, stop at 250 miles, pick up 250 (to make 1000), drop off 250 at 500 mile point, come back to plantation. (Status: 1000, 250,250,0) Camel ate 1500 bananas.
Last trip with 1000 bananas, stop at 250 miles, fill-up 250 (total 1000), stop at 500 miles, fill-up to make total 1000. Travel all the way to market with 500 left.
(Status: 0,0,0,500) Camel ate 500+1000+1000.
Net delivery: 500 bananas to market.