500
You have a fox, a chicken and a sack of grain. You must cross a river with only one of them at a time. If you leave the fox with the chicken, the fox will eat it. If you leave the chicken with the grain, the chicken will eat it. How can you get all three safely across?
First take the chicken. Then go back and bring over the grain. Bring back the chicken, leave the chicken and bring the fox. Then bring the chicken.