Sara should replace the first old washing machine with a new one which has an efficiency of 10 gallons per load.
An old washing machine uses = 35 gallons of water per load.
A new washing machine uses = 10 gallons of water per load.
Hence, the difference between the number of gallons used by the an old washing machine and a new washing machine's efficiency of using water is 15 gallons which will save more water.
Hence, sara should replace the first old washing machine with a new one which has an efficiency of 10 gallons per load.
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