An unknown radioactive element decays into non-radioactive substances. In 420 days the radioactivity of a sample decreases by 24 percent.
a) Find the exponential function that models this behavior
b) What is the half life of the element? (I already got the answer for this, and it is 1061 days)
c) How long for a sample of 100 mg to decay to 64 mg? (I already found the answer for this as well, and it is 683 days)
d) Graph the function in part a and be sure to include parts B and C in your graph
The only part I need help with is A and D. Thank you.