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9 because if you split the planta up into 10's, you would split the gallons into 3's. Thats 3 gallons per 10 plants
To do this, set up a proportion.  The proportion would be 6/20=x/30.  To do this, multiply 30 by 6.  This gives you 180. Now you want to divide 180 by 20. This gives you 9.  It would take 9 gallons of water to water 30 plants.
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