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A radioactive material disintegrates at a rate proportional to the amount currently present. If Q(t) is...

A radioactive material disintegrates at a rate proportional to the amount currently present. If Q(t) is the amount present at time t, then

dQ/dt =−rQ

where r>0 is the decay rate.

If 100 mg of a mystery substance decays to 81.14 mg in 4 weeks, find the time required for the substance to decay to one-half its original amount. Round the answer to 3 decimal places.

______________weeks

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