First find the total distance the potato flies in yards: 200 yards/field * 6 fields = 1200 yards
Then multiply that number by the number of feet per yard to convert it to feet: 1200 yards * 3 feet/yard = <<1200*3=3600>>3600 feet
Then divide that number by the dog's speed to find the number of minutes it has to run to fetch the potato: 3600 feet / 400 feet/minute = <<3600/400=9>>9 minutes
#### 9