The half-life of radioactive actinium (2Ac) is 21.77 years. What percent of a present amount of radioactive actinium will remain after 13 years? (Round your answer to the nearest percent.) % Need Help? Read It Submit Answer 9. [0/1 Points] DETAILS PREVIOUS ANSWERS LARPCALCLIM5 3.PS.014. MY NOTES ASK YOUR TEACHER PRACTICE ANOTHER A lab culture initially contains 500 bacteria. Four hours later, the number of bacteria decreases to 400. Find the exponential decay model of the form B = Boak that approximates the number of bacteria B in the culture after t hours. B= X Need Help? Read It