Elena has a washing machine with a 20 gallon capacity. She always uses a 20 gram measure of soap for every 10 gallons of water. One day she tells her son Francisco to fill the washing machine halfway with water, to add the soap and dissolve it well, but Francisco is wrong and adds 60 grams of soap to the water. When Elena realizes the mistake, she thinks of adding pure water to the tank at a rate of 2 gallons per minute and simultaneously opening the drain so that the volume of water in the washing machine remains constant. Assuming the liquid remains uniformly mixed, for how long should Elena add water to correct the error?