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Solve the following problems. 1. Find the half life of a radioactive substance if 25% of it disappears in 10 years. 2. A bacterial population B is known to have a rate of growth proportional to itself. If between noon and two in the aemoon the population triples, at what time , no control being exerted, should B becomes 100 times? 3. A glass of hot milk at 100C is brought in a room where the temperature is maintained at 20C. After 15 minutes the temperature goes down to 90C. Find: a. The temperature of the glass of milk after 15 more minutes. b. The time it takes until the temperature goes down to 37C. 4. A tank contains initially 2500 liters of 50% salt solution; water enters the tank at rate of 25 liters-'min. Find the percentage of salt in the tank aer 30 minutes