Answer:To find the half-life of the radioactive isotope, we can use the information given: 20.0 grams of the isotope are present at 1 p.m., and 5.0 grams remain at 2 p.m. The half-life of a radioactive substance is the amount of time it takes for half of the substance to decay or remain. In this case, the half-life represents the time it takes for 20.0 grams to decay to 10.0 grams. From 1 p.m. to 2 p.m., the isotope decreased from 20.0 grams to 5.0 grams, indicating that half of the original amount decayed. So, the half-life can be calculated as the time it took for the isotope to decay from 20.0 grams to 10.0 grams. To calculate the half-life, we need to know the specific decay rate or constant associated with the isotope. Without this information, we cannot determine the exact half-life. Therefore, based on the given information, we cannot determine the half-life of the isotope without additional data on the specific decay rate or constant.
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