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1. If 25% of a radioactive substance disappeared in 10 years, how long did it take for 613% of the substance to disappear? Hon.r many \" o disa ed in 5 years? 2. Initially, 22.?3 kg. of salt is dissolved in a large tank holding S gallons of water. A brine solution is pumped into the tank at a rate of 3 gallons per minute and well-stirred solution is then pumped out at the same rate. If the concentration of the solution entering is UH} pounds per gallon, determine the amount of salt in the tank at the end of l and a halfhour