First find how many corn plants there are: 15 plants/row * 4 rows = <<15*4=60>>60 plants
Then find how many gallons of water of the corn needs: 60 plants * 1/2 gallon/plant = <<60*1/2=30>>30 gallons
Then find how many gallons of water the pigs need: 10 pigs * 4 gallons/pig = <<10*4=40>>40 gallons
Then find how many gallons of water the ducks need: 20 ducks * 1/4 gallon/duck = <<20*1/4=5>>5 gallons
Now add up all three quantities of water to find the total water needed: 5 gallons + 40 gallons + 30 gallons = <<5+40+30=75>>75 gallons
Now divide the total water needed by Casey's pumping rate to find how long she spends pumping: 75 gallons / 3 gallons/minute = <<75/3=25>>25 minutes
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