Need help!! A certain radioactive substance decays exponentially over time. The amount of a
sample of the substance that remains, P, after t years is given by P(t) = 10e^-47,
where P is expressed as a percentage.
a. At what time has 50% of the substance decayed? (5 marks)
b. Determine the rate of change of the function. (2 marks)
c. What is the rate of decay when 50% of the original sample has decayed? (2 marks)

show calculation pls. can you answer any of the question?