Ican't figure this out. penn foster

Apatient is given 50 mg dose of medicine the medicines effectiveness decreases every hour at a constant rate of 40% what is the exponential decay function that models this scenario how much medicine will be left in the patients system after 2 hours

Choose a second initial value that is 0.01 greater than the initial value from question 9. iterate it using the function, f, ten times. if necessary, you can round your results to the nearest ten-thousandth.

Math i have no idea how to do a and b