10. A radioactive material, such as the isotope thorium-234, disintegrates at a rate propor- tional to the amount currently present. If C206) is the amount present at a time t, then dQ_ 5"\"; where "r > 0 is the decay rate. a. If 100 mg of Thorium-234 decays to 82.04 mg in 1 week, determine the decay rate r. 1). Find an expression for the amount of thorium-234 present at any time t. c. Find the time required for the thorium234 to decay to onehaif its original amount