Ok...he'd have 5 watermelons when he got to the fair.

He takes his first 15 watermelons and walks 1 mile. When he gets to his first mile, he has 14 left. He drops 13 and takes one back with him to eat on his return trip. He does this for two more trips for his other 30 watermelons. Now at his first mile, he has 39 watermelons.

He takes 13 watermelons and walks another mile (so now he is at mile 2). He drops 11 and takes one for the walk back. He does this two more times. At his second mile, he has 33 watermelons.

He takes 11 watermelons and walks another mile (so now he is at mile 3). He drops 9 and takes one for the return trip. He does this two more times. At his third mile, he has 27 watermelons.

He takes 9 watermelons and walks another mile (he is now at mile 4), drops 7 an takes one for the return trip. After two more trips like this, he has 21 watermelons at mile 4.

He takes 7 watermelons and walks one more mile. He drops 5 and takes one for the return trip. After two more trips, he now has 15 watermelons at mile number 5.

Since he has 15 watermelons now, he can walk the last 10 miles, eat one every mile and arrive at the fair with 5 uneaten, but probably very warm and possibly rotten watermelons! AND he has walked a total of 100 miles!!!